--- a/services/sync/modules/engines.js
+++ b/services/sync/modules/engines.js
@@ -485,16 +485,22 @@ SyncEngine.prototype = {
           this._tracker.ignoreAll = true;
           this._store.applyIncoming(item);
         } else {
           count.reconciled++;
           this._log.trace("Skipping reconciled incoming item " + item.id);
         }
       }
       catch(ex) {
+
+        if (!Utils.isHMACMismatch(ex)) {
+          // Rethrow anything we shouldn't handle.
+          throw ex;
+        }
+
         this._log.warn("Error processing record: " + Utils.exceptionStr(ex));
 
         // Upload a new record to replace the bad one if we have it
         if (this._store.itemExists(item.id))
           this._modified[item.id] = 0;
       }
       this._tracker.ignoreAll = false;
       Sync.sleep(0);