One of my users is also seeing this error on our live site. Nothing is showing up in the unhandled exception logging that we have setup for the site (Global.asax > Application_Error). I've yet to get any diagnostic information from the user, but I will add that when I do. It's on a pair of load-balanced servers. I am not seeing anything in the windows Event log either. I tried reproducing, and I am not getting the error. 

If you have any advice on how to catch these errors as they occur; besides contacting end users individually, please let me know.

Thanks,

Colin