NUnit 2.5 up to and including 2.5.4 fails to properly detect certain revisions of .NET 4.0, causing it to crash. The issue is documented in this bug report on LaunchPad. The bug has been fixed in 2.5.5 and later, so go grab the latest version of NUnit 2.5 and run your tests smoothly again, preferably on each build using Continuous Testing, of course. 