Story time: years ago (2010-ish?), we were doing some light web CMS work for a client. Nothing too complex, except we had to regenerate one section off a data feed that we received in a very peculiar custom format.

Great, everything worked, we finished on time and on budget. And then: "oh, and could you also check this other site? We need some minor tweaks on what your predecessors made". That other site was also consuming that data feed, so we took a peek. It ran in PHP3, walled off from everything, processed its own intermediate languageS, and the output was a massive 200+ page PDF (which was then manually shipped to offset print in large quantities). For Reasons, this had to run daily and had to work on first try. There was no documentation, no comments, and no development environment: apparently this was made directly on prod and carefully left untouched.

Needless to say, the code was massively interdependent, so that the minor tweaks were actually somewhat harder. We did manage to set up a development VM though, and document it a bit - but last I checked, the Monstrous Book Of Monsters Generator still seems to be chugging along in its containment.