diff --git a/src/repository/parser/ArcanistMercurialParser.php b/src/repository/parser/ArcanistMercurialParser.php
index 8317cc7c..4eb10e2f 100644
--- a/src/repository/parser/ArcanistMercurialParser.php
+++ b/src/repository/parser/ArcanistMercurialParser.php
@@ -174,6 +174,10 @@ final class ArcanistMercurialParser extends Phobject {
           case 'bookmark':
             $commit['bookmark'] = $value;
             break;
+          case 'obsolete':
+            // This is an extra field added by the "evolve" extension even
+            // if HGPLAIN=1 is set. See PHI502.
+            break;
           default:
             throw new Exception(
               pht("Unknown Mercurial log field '%s'!", $name));
diff --git a/src/workflow/ArcanistFeatureWorkflow.php b/src/workflow/ArcanistFeatureWorkflow.php
index 99a28200..a1cbe108 100644
--- a/src/workflow/ArcanistFeatureWorkflow.php
+++ b/src/workflow/ArcanistFeatureWorkflow.php
@@ -167,7 +167,7 @@ EOTEXT
     }
 
     echo $stdout;
-    fprintf(STDERR, $stderr);
+    fprintf(STDERR, '%s', $stderr);
     return $err;
   }