Worst experience?
Fourth semester. Programming project.
We were 4 persons, I did almost all the work (including fixing the stuff they broke) but that's not the point.

One of them somehow killed our git repo at least once per weak. It was really annoying, because I had to fix it.

Also he named *all* of his commits 'Pfuschpush' (Pfusch is German for botch). And the code had exactly this quality. I often had to rewrite everything he did (or simply revert the commit).