There was a big hairy ball of SW mud from another project that a poor coworker had to "reuse". Only that it was impossible because there was no documentation, shit was partly auto-generated with mysterious Excel tables, and the actual code was just as bad. No APIs and nothing, just hacking shit into globals, several nested state machines that were overriding each other's states, and with global side effects. WTF.

Two devs took a look at it - minimum 8 weeks. Schedule was some days, and PM insisted that it was "already working". But the worst thing was that the dev in charge had been looking for another job anyway and quit, so the whole clusterfuck suddenly was on my desk.

The code was so awful that I could only bear it with both eyes closed, so I instead read the spec of this project closely. Turned out that it didn't actually demand this feature, only a small subset of what the ball of mud was supposed to achieve - which I was able to implement from scratch within a day, plus another one for documentation. Phew.