Why does the custom error handling not work for ".aspx."(aspx+period)?

My custom error handling is broken for aspx+period requests.

ALSO for MSDN's default.aspx!!! Add a period after the x

This is on only one of multiple production sites. The MSDN site gives the exact same error.

Runtime error:

An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. the request has been terminated.