Probably a kernel module for Linux that involved non-documented hardware with reverse engineering and fumbling around in the dark, essentially.

Last year I got asked to add some functionality to a commercial app. The app.... was Implemented as cgi scripts written in awk. There's a backed c app often called, if you call with the wrong input it just segfaults.

"Hardest" to me has little to do with algorithms in the eyes of my recurrent nightmares about this abomination.

A web app in Gatsby. They force you to throw away half of what you know about programming and rely on their crappy mental models for developing websites

I had to transcribe and improve a program to calculate the correlation coefficient. The source was a very poor photocopy and the code had many i s' and 1's which were nearly impossible to distinguish.