VB apps running on IIS is up there; I'm happy to never deal with either again. This was pre-dot-net.
Early 2000s doing tech support in a house-built ticketing+crm system that heavily abused things in JS.
Speaking of tech support, all manner of software mentioned here already.
I spent a ton of time working in healthcare IT and there's all kinds of janky mess there. One was a house-built Perl script to handle certain things. It was like 15k lines (and before you blame Perl, we had a Java class that was over 30k IIRC). Never allowed to rewrite it because of how mission-critical it was, yet there were still bugs with it. Healthcare IT in the US tends to have lots of jank, especially the small clinics that had to start by doing everything they could with what they had.
I don't want to leave my current job, but they make us all use Mac (Apple Silicon) for software engineering and I hate it. Nothing works the way I expect, it's not consistent between apps, certain ML tools we use won't work on it (mostly because not x86 arch), etc.