diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 7a620a74..467d61e1 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -33,7 +33,9 @@ phutil_register_library_map(array(
     'ArcanistDiffWorkflow' => 'workflow/diff',
     'ArcanistDifferentialCommitMessage' => 'differential/commitmessage',
     'ArcanistDifferentialCommitMessageParserException' => 'differential/commitmessage',
+    'ArcanistDifferentialRevisionHash' => 'differential/constants/revisionhash',
     'ArcanistDifferentialRevisionRef' => 'differential/revision',
+    'ArcanistDifferentialRevisionStatus' => 'differential/constants/revisionstatus',
     'ArcanistDownloadWorkflow' => 'workflow/download',
     'ArcanistEventType' => 'events/constant/type',
     'ArcanistExportWorkflow' => 'workflow/export',
diff --git a/src/differential/constants/revisionhash/ArcanistDifferentialRevisionHash.php b/src/differential/constants/revisionhash/ArcanistDifferentialRevisionHash.php
new file mode 100644
index 00000000..cbbc43d0
--- /dev/null
+++ b/src/differential/constants/revisionhash/ArcanistDifferentialRevisionHash.php
@@ -0,0 +1,35 @@
+<?php
+
+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+final class ArcanistDifferentialRevisionHash {
+
+  const TABLE_NAME = 'differential_revisionhash';
+
+  const HASH_GIT_COMMIT         = 'gtcm';
+  const HASH_GIT_TREE           = 'gttr';
+  const HASH_MERCURIAL_COMMIT   = 'hgcm';
+
+  public static function getTypes() {
+    return array(
+      ArcanistDifferentialRevisionHash::HASH_GIT_COMMIT,
+      ArcanistDifferentialRevisionHash::HASH_GIT_TREE,
+      ArcanistDifferentialRevisionHash::HASH_MERCURIAL_COMMIT,
+    );
+  }
+
+}
diff --git a/src/differential/constants/revisionhash/__init__.php b/src/differential/constants/revisionhash/__init__.php
new file mode 100644
index 00000000..fdbebe93
--- /dev/null
+++ b/src/differential/constants/revisionhash/__init__.php
@@ -0,0 +1,10 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+
+phutil_require_source('ArcanistDifferentialRevisionHash.php');
diff --git a/src/differential/constants/revisionstatus/ArcanistDifferentialRevisionStatus.php b/src/differential/constants/revisionstatus/ArcanistDifferentialRevisionStatus.php
new file mode 100644
index 00000000..5b9ec4c6
--- /dev/null
+++ b/src/differential/constants/revisionstatus/ArcanistDifferentialRevisionStatus.php
@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * Copyright 2012 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+final class ArcanistDifferentialRevisionStatus {
+
+  const NEEDS_REVIEW      = 0;
+  const NEEDS_REVISION    = 1;
+  const ACCEPTED          = 2;
+  const COMMITTED         = 3;
+  const ABANDONED         = 4;
+
+  public static function getNameForRevisionStatus($status) {
+    static $map = array(
+      self::NEEDS_REVIEW      => 'Needs Review',
+      self::NEEDS_REVISION    => 'Needs Revision',
+      self::ACCEPTED          => 'Accepted',
+      self::COMMITTED         => 'Committed',
+      self::ABANDONED         => 'Abandoned',
+    );
+
+    return idx($map, coalesce($status, '?'), 'Unknown');
+  }
+
+}
diff --git a/src/differential/constants/revisionstatus/__init__.php b/src/differential/constants/revisionstatus/__init__.php
new file mode 100644
index 00000000..e407b4c9
--- /dev/null
+++ b/src/differential/constants/revisionstatus/__init__.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('phutil', 'utils');
+
+
+phutil_require_source('ArcanistDifferentialRevisionStatus.php');
diff --git a/src/parser/bundle/ArcanistBundle.php b/src/parser/bundle/ArcanistBundle.php
index 5f9811ca..1599e4eb 100644
--- a/src/parser/bundle/ArcanistBundle.php
+++ b/src/parser/bundle/ArcanistBundle.php
@@ -28,6 +28,7 @@ class ArcanistBundle {
   private $blobs = array();
   private $diskPath;
   private $projectID;
+  private $baseRevision;
 
   public function setConduit(ConduitClient $conduit) {
     $this->conduit = $conduit;
@@ -41,6 +42,14 @@ class ArcanistBundle {
     return $this->projectID;
   }
 
+  public function setBaseRevision($base_revision) {
+    $this->baseRevision = $base_revision;
+  }
+
+  public function getBaseRevision() {
+    return $this->baseRevision;
+  }
+
   public static function newFromChanges(array $changes) {
     $obj = new ArcanistBundle();
     $obj->changes = $changes;
@@ -65,10 +74,12 @@ class ArcanistBundle {
       $meta_info = $future->resolveJSON();
       $version = idx($meta_info, 'version', 0);
       $project_name = idx($meta_info, 'projectName');
+      $base_revision = idx($meta_info, 'baseRevision');
     // this arc bundle was probably made before we started storing meta info
     } else {
       $version = 0;
       $project_name = null;
+      $base_revision = null;
     }
 
     $future = new ExecFuture(
@@ -92,6 +103,7 @@ class ArcanistBundle {
     $obj->changes = $changes;
     $obj->diskPath = $path;
     $obj->setProjectID($project_name);
+    $obj->setBaseRevision($base_revision);
 
     return $obj;
   }
@@ -138,8 +150,11 @@ class ArcanistBundle {
       $blobs[$phid] = $this->getBlob($phid);
     }
 
-    $meta_info = array('version' => 1,
-                       'projectName' => $this->getProjectID());
+    $meta_info = array(
+      'version'      => 2,
+      'projectName'  => $this->getProjectID(),
+      'baseRevision' => $this->getBaseRevision(),
+    );
 
     $dir = Filesystem::createTemporaryDirectory();
     Filesystem::createDirectory($dir.'/hunks');
diff --git a/src/repository/api/base/ArcanistRepositoryAPI.php b/src/repository/api/base/ArcanistRepositoryAPI.php
index c447c5d8..e5a280e5 100644
--- a/src/repository/api/base/ArcanistRepositoryAPI.php
+++ b/src/repository/api/base/ArcanistRepositoryAPI.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -151,9 +151,9 @@ abstract class ArcanistRepositoryAPI {
   abstract public function getOriginalFileData($path);
   abstract public function getCurrentFileData($path);
   abstract public function getLocalCommitInformation();
-
-
+  abstract public function getSourceControlBaseRevision();
   abstract public function supportsRelativeLocalCommits();
+  abstract public function getWorkingCopyRevision();
 
   public function parseRelativeLocalCommit(array $argv) {
     throw new ArcanistCapabilityNotSupportedException($this);
diff --git a/src/repository/api/git/ArcanistGitAPI.php b/src/repository/api/git/ArcanistGitAPI.php
index fd50d2eb..b27ba143 100644
--- a/src/repository/api/git/ArcanistGitAPI.php
+++ b/src/repository/api/git/ArcanistGitAPI.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -544,6 +544,14 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
     return trim($owner);
   }
 
+  public function getWorkingCopyRevision() {
+    list($stdout) = execx(
+      '(cd %s; git rev-parse %s)',
+      $this->getPath(),
+      'HEAD');
+    return rtrim($stdout, "\n");
+  }
+
   public function supportsRelativeLocalCommits() {
     return true;
   }
diff --git a/src/repository/api/mercurial/ArcanistMercurialAPI.php b/src/repository/api/mercurial/ArcanistMercurialAPI.php
index 127df939..fd893b42 100644
--- a/src/repository/api/mercurial/ArcanistMercurialAPI.php
+++ b/src/repository/api/mercurial/ArcanistMercurialAPI.php
@@ -304,7 +304,7 @@ class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     }
   }
 
-  private function getWorkingCopyRevision() {
+  public function getWorkingCopyRevision() {
     // In Mercurial, "tip" means the tip of the current branch, not what's in
     // the working copy. The tip may be ahead of the working copy. We need to
     // use "hg summary" to figure out what is actually in the working copy.
diff --git a/src/repository/api/subversion/ArcanistSubversionAPI.php b/src/repository/api/subversion/ArcanistSubversionAPI.php
index e73a1e37..9656d17a 100644
--- a/src/repository/api/subversion/ArcanistSubversionAPI.php
+++ b/src/repository/api/subversion/ArcanistSubversionAPI.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -492,6 +492,10 @@ EODIFF;
     return false;
   }
 
+  public function getWorkingCopyRevision() {
+    return $this->getSourceControlBaseRevision();
+  }
+
   public function supportsLocalBranchMerge() {
     return false;
   }
diff --git a/src/workflow/base/ArcanistBaseWorkflow.php b/src/workflow/base/ArcanistBaseWorkflow.php
index 6bcb5375..1400263a 100644
--- a/src/workflow/base/ArcanistBaseWorkflow.php
+++ b/src/workflow/base/ArcanistBaseWorkflow.php
@@ -752,6 +752,7 @@ class ArcanistBaseWorkflow {
     $bundle = ArcanistBundle::newFromChanges($changes);
     $bundle->setConduit($conduit);
     $bundle->setProjectID($diff['projectName']);
+    $bundle->setBaseRevision($diff['sourceControlBaseRevision']);
     return $bundle;
   }
 
diff --git a/src/workflow/export/ArcanistExportWorkflow.php b/src/workflow/export/ArcanistExportWorkflow.php
index ad39e9ff..ff9b5daf 100644
--- a/src/workflow/export/ArcanistExportWorkflow.php
+++ b/src/workflow/export/ArcanistExportWorkflow.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -187,6 +187,8 @@ EOTEXT
 
         $bundle = ArcanistBundle::newFromChanges($changes);
         $bundle->setProjectID($this->getWorkingCopy()->getProjectID());
+        $bundle->setBaseRevision(
+          $repository_api->getSourceControlBaseRevision());
         break;
       case self::SOURCE_REVISION:
         $bundle = $this->loadRevisionBundleFromConduit(
diff --git a/src/workflow/patch/ArcanistPatchWorkflow.php b/src/workflow/patch/ArcanistPatchWorkflow.php
index f28303ec..eaaa100f 100644
--- a/src/workflow/patch/ArcanistPatchWorkflow.php
+++ b/src/workflow/patch/ArcanistPatchWorkflow.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -135,8 +135,7 @@ EOTEXT
   }
 
   public function requiresConduit() {
-    return ($this->getSource() == self::SOURCE_REVISION) ||
-           ($this->getSource() == self::SOURCE_DIFF);
+    return ($this->getSource() != self::SOURCE_PATCH);
   }
 
   public function requiresRepositoryAPI() {
@@ -428,12 +427,12 @@ EOTEXT
    */
   private function sanityCheckPatch(ArcanistBundle $bundle) {
 
-    // Check to see if the bundle project id matches the working copy
+    // Check to see if the bundle's project id matches the working copy
     // project id
     $bundle_project_id = $bundle->getProjectID();
     $working_copy_project_id = $this->getWorkingCopy()->getProjectID();
     if (empty($bundle_project_id)) {
-      // this means $source is SOURCE_PATCH || SOURCE_BUNDLE
+      // this means $source is SOURCE_PATCH || SOURCE_BUNDLE w/ $version = 0
       // they don't come with a project id so just do nothing
     } else if ($bundle_project_id != $working_copy_project_id) {
       $ok = phutil_console_confirm(
@@ -447,6 +446,69 @@ EOTEXT
       }
     }
 
+    // Check to see if the bundle's base revision matches the working copy
+    // base revision
+    $bundle_base_rev = $bundle->getBaseRevision();
+    if (empty($bundle_base_rev)) {
+      // this means $source is SOURCE_PATCH || SOURCE_BUNDLE w/ $version < 2
+      // they don't have a base rev so just do nothing
+    } else {
+      $repository_api = $this->getRepositoryAPI();
+      $source_base_rev = $repository_api->getWorkingCopyRevision();
+
+      if ($source_base_rev != $bundle_base_rev) {
+        // we have a problem...! lots of work because we need to ask
+        // differential for revision information for these base revisions
+        // to improve our error message.
+        $bundle_base_rev_str = null;
+        $source_base_rev_str = null;
+
+        // SVN doesn't store these hashes, so we're basically done already
+        // and will have a relatively "lame" error message
+        if ($repository_api instanceof ArcanistSubversionAPI) {
+          $hash_type = null;
+        } else if ($repository_api instanceof ArcanistGitAPI) {
+          $hash_type = ArcanistDifferentialRevisionHash::HASH_GIT_COMMIT;
+        } else if ($repository_api instanceof ArcanistMercurialAPI) {
+          $hash_type = ArcanistDifferentialRevisionHash::HASH_MERCURIAL_COMMIT;
+        } else {
+          $hash_type = null;
+        }
+
+        if ($hash_type) {
+          // 2 round trips because even though we could send off one query
+          // we wouldn't be able to tell which revisions were for which hash
+          $hash = array($hash_type, $bundle_base_rev);
+          $bundle_revision = $this->loadRevisionFromHash($hash);
+          $hash = array($hash_type, $source_base_rev);
+          $source_revision = $this->loadRevisionFromHash($hash);
+
+          if ($bundle_revision) {
+            $bundle_base_rev_str = $bundle_base_rev .
+                                   ' \ D' . $bundle_revision['id'];
+          }
+          if ($source_revision) {
+            $source_base_rev_str = $source_base_rev .
+                                   ' \ D' . $source_revision['id'];
+          }
+        }
+        $bundle_base_rev_str = nonempty($bundle_base_rev_str,
+                                        $bundle_base_rev);
+        $source_base_rev_str = nonempty($source_base_rev_str,
+                                        $source_base_rev);
+
+        $ok = phutil_console_confirm(
+          "This diff is against commit {$bundle_base_rev_str}, but the ".
+          "working copy is at {$source_base_rev_str}.  ".
+          "Still try to apply it?",
+          $default_no = false
+        );
+        if (!$ok) {
+          throw new ArcanistUserAbortException();
+        }
+      }
+    }
+
     // TODO -- more sanity checks here
   }
 
@@ -473,4 +535,27 @@ EOTEXT
           $dir));
     }
   }
+
+  private function loadRevisionFromHash($hash) {
+    $conduit = $this->getConduit();
+
+    $revisions = $conduit->callMethodSynchronous(
+      'differential.query',
+      array(
+        'commitHashes' => array($hash),
+      )
+    );
+
+
+    // grab the latest committed revision only
+    $found_revision = null;
+    $revisions = isort($revisions, 'dateModified');
+    foreach ($revisions as $revision) {
+      if ($revision['status'] ==
+          ArcanistDifferentialRevisionStatus::COMMITTED) {
+        $found_revision = $revision;
+      }
+    }
+    return $found_revision;
+  }
 }
diff --git a/src/workflow/patch/__init__.php b/src/workflow/patch/__init__.php
index 3449002a..35015c6f 100644
--- a/src/workflow/patch/__init__.php
+++ b/src/workflow/patch/__init__.php
@@ -6,6 +6,8 @@
 
 
 
+phutil_require_module('arcanist', 'differential/constants/revisionhash');
+phutil_require_module('arcanist', 'differential/constants/revisionstatus');
 phutil_require_module('arcanist', 'exception/usage');
 phutil_require_module('arcanist', 'exception/usage/userabort');
 phutil_require_module('arcanist', 'parser/bundle');