Strange Request URLs in logged ASP.NET exceptions, but not in IIS logs

Question

Getting unhandled exception event log messages for legitimate exceptions but the event log message includes noise in the request URL.

The noise is injected where uri escaped characters are. Noise like the apppool name, "An unhandled exception has occurred.", False and sometimes the request url itself recursively injected, sometimes a dozen times inside the url. The IIS logs don't show this injection, just the original url.

Here's one of the recursive examples:

Request URL: http://domain.com/Page.aspx?q=bransonAn unhandled exception has occurred.Chttp://domain.com/Page.aspx?q=bransonAn unhandled exception has occurred.Chttp://domain.com/Page.aspx?q=bransonAn unhandled exception has occurred.Chttp://domain.com/Page.aspx?q=branson%2C%20momomomo

The one somewhat unusual thing about the code is that it uses an exception filter which does some logging and then returns false, allowing the exception to go unhandled.