Code fuckup day or what?! After two weeks where I wasn't on my project and a co-worker handled it, I came back to my project and reviewed what he had done so far.
Me: "I don't understand how this new code part here can work?"
Him: "Uhm, actually, it doesn't, somehow."
Me: "..."
Then he had checked in his stuff with spaces while the whole project is with tabs. And variables that were used in a different way, but still under the old name, now completely misleading. Bypassing existing infrastructure and defines with "just for this case" hacks. But the best was tracking higher level state by peeking into lower level data buffers, even pulling out their data definitions into global header files - instead of using proper states in the higher layer itself.
NOT! IN! MY! FUCKING! PROJECT!!!
So I spent the day cleaning up the shit to fight off software rot right in the beginning.