Yesterday I had a client report an issue that I needed to debug locally. This particular piece of functionality required a wildcard mapping to intercept and inspect all 404 errors. The production site is running on IIS 6 in 32 bit mode. My local development environment has the site running in classic mode on IIS 7. So when attempting to setup the wildcard mapping locally windows will by default open up the aspnet_isapi.dll found int the C:\Windows\Microsoft.NET\Framework\v2.0.50727 folder. This is fine if you are running your site in 32 bit mode, however this is not the aspnet_isapi.dll used if your site is running under 64 bit mode. Instead you should reference the aspnet_isapi.dll found in C:\Windows\Microsoft.NET\Framework64\v2.0.50727. I hope this saves you the time and headache it took me to realize this was my problem.