diff -u b/misc/ajax.js b/misc/ajax.js
--- b/misc/ajax.js
+++ b/misc/ajax.js
@@ -476,11 +476,13 @@
  * Handler for the form redirection error.
  */
 Drupal.ajax.prototype.error = function (xmlhttprequest, uri, customMessage) {
-  if (xmlhttprequest.status) {
-    alert(Drupal.ajaxError(xmlhttprequest, uri, customMessage));
-  }
-  else if (window.console) {
-    console.log(Drupal.ajaxError(xmlhttprequest, uri, customMessage));
+  if (xmlhttprequest.status >= 400) {
+    if (window.console) {
+      console.log(Drupal.ajaxError(xmlhttprequest, uri, customMessage));
+    }
+    else {
+      alert(Drupal.ajaxError(xmlhttprequest, uri, customMessage));
+    }
   }
   // Remove the progress element.
   if (this.progress.element) {
diff -u b/misc/autocomplete.js b/misc/autocomplete.js
--- b/misc/autocomplete.js
+++ b/misc/autocomplete.js
@@ -310,12 +310,15 @@
         }
       },
       error: function (xmlhttp) {
-        if (xmlhttp.status) {
-          alert(Drupal.ajaxError(xmlhttp, db.uri));
-        }
-        else if (window.console) {
-          console.log(Drupal.ajaxError(xmlhttp, db.uri));
+        if (xmlhttp.status >= 400) {
+          if (window.console) {
+            console.log(Drupal.ajaxError(xmlhttp, db.uri));
+          }
+          else {
+            alert(Drupal.ajaxError(xmlhttp, db.uri));
+          }
         }
+
       }
     });
   }, this.delay);