I encountered a bug using Machine.Specifications where certain test specifications fail. I believe it has to do with using a type name in the specification name followed by "_s", but that is pure speculation. Using a capital "S" makes the test pass. I have included a simple example with passing and failing specifications in the Gist linked below. The tests pass in all other runners (ReSharper mainly) that I have tried.

Bug Repro Gist

Thanks for looking into it. Hope this helps someone else, it was a strange one to diagnose.

For anyone interested, a fix for this problem has been introduced with the newly released version of NCrunch (v1.45).