I've just had a weird problem on the build server. For one of the test projects, it would not build at all. The error message was the following:
Apparently, no NUnit library could be found. Checking the GAC manually, this appears to be true. How weird. Tried reinstalling and repairing NUnit 2.5.5 a few times with no result. Then, in Visual Studio Command Prompt, adding NUnit dll to GAC manually with
solved the issue.
Reference:How to copy/install assembly in a GAC? (Some problem running NUnit) - Stack Overflow