Parts of the code I am working on date back to the early nineties, written in ancient C++ with lots of special cases for ancient compilers by people with 0-2 years of coding experience.
My favourite coding moment is every time when after refactoring a part of the code, it has about 1000 lines less (no exaggeration), is more reliable, AND can do a lot more than before.

Fucking MikTex.

Apparently this software got stuck somewhere back in the nineties. Got a fresh installation to make sure issues were not arising from outdated hardware - still connection errors, API errors, horrible, unresponsive and ugly UI, what a broken piece of ****.

Package build fails on Jenkins. I try to investigate locally. Builds fine locally. Managed to convince admin to let me ssh there to debug there in the exactly same environment. Builds there without any issues when triggered manually. I'm missing some little piece of environment from jenkins job or there could be some race somewhere. I really hate those random errors, we really need to get sane build system so I wouldn't need to debug those irreproducible build issues. Aaarrrghh!!!