Unhandled GZip Encoding
What happens when you have GZip turned on and there is an unhandled exception is thrown, look no further.. a completely cryptic garbage page is returned
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So whats the problem, well when an unhandled exception is thrown ASP.NET/MVC removes the GZip header or any other custom header that was set and returns a non-zipped content while your IIS decides to send to browser that the content is GZipped when it's not really. So your browser is receiving a non-zipped content with GZIP content type, thus the garbage content.
To fix this in the global.ascx add the following to the Application_PreSendRequestHeaders function: