It looks like an ajax update is occurring for the element that is the source of the event.  In the past we have resolved this with the following strategy:
+++ src/main/javascript/application.js	(working copy)
@@ -108,6 +108,15 @@
         }
         function formOf(element) {
+            try 
{
+                return formOfNode(element);
+            }
 catch (e)  
{
+                //page update may have occurred
+                return formOfNode(document.getElementById(element.id));
+            }
+        }
+
+        function formOfNode(element) 
{
             return toLowerCase(element.nodeName) == 'form' ? element : enclosingForm(element);
         }
This fixes the test case.  In the past, DOM operations in the middle of ajax updates have been found to be highly browser-dependent.
  
    
The stopBlockingUI callback defined to run on onAfterUpdate is never executed.
The pre-request callback to setup the stopBlockingUI callback runs correctly though.