R12.1.3: Workflow error after RPC2

In My Oracle Support document "You Have Encountered An Error In The Worklist Notifications" from note ID 1946243.1 we learn that after applying Recommended Patch Collection 2 (and the prerequisites) an error message including "OANullDBTransactionImpl cannot be cast" is encountered.  What is the problem here?  It seems that this is related to a browser issue, so plan ahead now for when you have to apply RPC2.