TL;DR: At a house party, on my Phone, via shitty German mobile network using the GitLab website's plain text editor. Thanks to CI/CD my changes to the code were easily tested and deployed to the server.

It was for a college project and someone had a bug in his 600+ lines function that was nested like hell. At least 7 levels deep. Told him before I went to that party it's probably a redefined counter variable but he wouldn't have it as he was sure it was an error with the business logic. Told him to simplify the code then but he wouldn't do that either because "the code/logic is too complex to be simplified"... Yeah... what a dipshit...
Nonetheless I went to the party and He kept debugging. At some point he called me and asked me to help him the following day. Knowing that the code had to be fixed anyways I agreed.
I also knew I wouldn't be much of a help the next day due to side effects of the party, so I tried looking at this shitshow of a function on my phone. Oh did I mention it was PHP, yet? Yeah... About 30 minutes and a beer later I found the bug and of course it was a redefined counter variable... My respect for him as a dev was already crumbling but it died completely during that evening