Was helping an intern with testing and he was trying to hit a specific branch in the code and just couldn't get there. Spent probably 20 minutes trying to figure out what the hell was going on since by all reasonable logic the code should run. Getting other developers involved trying to figure out what the hell was going on. Then discovering he was using a stub of the class with the method we were trying to reach overwritten to simply return null. Pretty much just wanted to go on a walk at that point.