diff --git a/src/applications/diffusion/controller/DiffusionServeController.php b/src/applications/diffusion/controller/DiffusionServeController.php
--- a/src/applications/diffusion/controller/DiffusionServeController.php
+++ b/src/applications/diffusion/controller/DiffusionServeController.php
@@ -397,7 +397,25 @@
       switch ($vcs_type) {
         case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-          $result = $this->serveVCSRequest($repository, $viewer);
+          $caught = null;
+          try {
+            $result = $this->serveVCSRequest($repository, $viewer);
+          } catch (Exception $ex) {
+            $caught = $ex;
+          } catch (Throwable $ex) {
+            $caught = $ex;
+          }
+
+          if ($caught) {
+            // We never expect an uncaught exception here, so dump it to the
+            // log. All routine errors should have been converted into Response
+            // objects by a lower layer.
+            phlog($caught);
+
+            $result = new PhabricatorVCSResponse(
+              500,
+              phutil_string_cast($caught->getMessage()));
+          }
           break;
         case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
           $result = new PhabricatorVCSResponse(