I recently started using VirtualBox as a replacement for VirtualPC 2007 to do builds using an old build environment. The NUnit test was failing, so I fixed the problem, compiled, and reran the test. Hmmm, still failing! Retried a couple of times, and then it hit me! NUnit didn't see the changed binaries when in VirtualBox. So I restarted NUnit, and presto! Tests are running again! Now I hope I won't forget next time...