I have an issue where adding this to any asp.net request causes and unhandled error (regardless of CustomErrors setting): ?aspxerrorpath=<script The weird thing is that I have two applications that are totally immune to the problem, but I can't spot the difference. I'm running on IIS 7 and using the 4.0 Framework. But this also happens on my 3.5 IIS 6 apps. Again, I have two sites that are fine (IIS 7, 4.0) but can't determine why. I have tried turning off request validation to no avail. Has anyone encountered something similar? FIXED: This looks like it was an Umbraco issue. It was ignoring my validateRequest="false" in the web.config. Adding this to the template fixed it: