Lately we've been experiencing one of the downsides of all the tooling we've put into our continuous integration practice. It's fragile. Trying to get all your tools lined up with versions that all get along is tricky, and hard to keep balanced. Particularly when two of them (NCover and TypeMock) both try to register themselves as profiles, and have to be chained to keep the tests running while gathering coverage information.
I think we've just about got everybody moved up to the right versions, but there's been lots of "it works on my machine" the last week or so.
The price you pay, I guess.