Classic unit testing mistake!
Got into the office this morning after a day out of the office and started hacking away at some code. This particular bit of code was quite a way in. Finished what I was doing... ran my tests and... excellent... all green Run the entire suite for the project and nearly all of them went red.
Then I wondered, “did I do that, or where they all broken yesterday when I wasn't here?”
The moral of the story (and one that I'm reliably informed by my friend Tony I will continually relearn) is: run all the tests before you start hacking the code so you know if you broke them.