I had a similar story yesterday at work.
I mentioned prior that I'm a Django dev. Well, we began using qunit to unit test our javascript a month or so back. I'm the only one on the team besides the lead that gives a damn about them. In fact, the commit logs show that every single line of code was written by yours truly. I'm not saying I like qunit as a framework and working with javascript all day gives me a throbbing headache, but it's my misshapen, diseased baby.
So, when one of our new hires made some pretty sweeping changes to the logic (which was desperately needed, don't get me wrong), the tests broke. Those broken tests got merged because I'm just about the only one that checks them when working on the front-end.
Since I was working on the module anyway, I tried fixing the tests for him. The new hire's changes involved making certain display elements change colors when a specific event takes place (how's that for anonymization of our prodcut? Colors change when a button is pressed), I set up a fixture that roughly approximates the structure in production and began testing that the color of elements were what they should be before and after the event. Only it didn't work. I ran through our code for half of the day and could find absolutely nothing wrong. I hand-traced the path through our mess of javascript. I manually examined the document structure of both the actual page and my test fixture and could find no significant differences that would prevent this from working. In fact, my debugging told me that it was working just fine.
Fast forward to a few minutes before I leave, when I realize that everything was working perfectly. Our css files were not included in the test because we're supposed to be testing javascript, not css.