We had this problem a couple of times, too.  What's interesting is our site uses a different set of user controls based on the item a customer is purchasing.  When the app went down, only one product was causing the error.  After reading this thread, I don't think it's coincidental that the product having the issue was using a nested user control, while the product not having the problem had a flat single user control structure.

In our case, copying the user control from Test to Prod fixed the problem the first couple of times.  Since then, we've replaced the nested user control with a flat structure.  We haven't seen the issue for about a week.  Not sure if it's been fixed, or just hasn't errored yet...