The Application_Error method is called only AFTER a finally block, in case you have a try finally.

Therefore - if a exception occurs in the finally statement (in the dispose method) then the previous exception wasn't dispatched to the Application_Error on global.asax yet, and by the time the framework calls the Application_Error the finally statement has already ran and throwed it's own exception, so when you call Server.GetLastError() you'll  get only the exception from the finally statement!