Thats pretty tame. I once got a bug like: Some contractor wrote this, got mgmt to get a new, insane deployment procedure scripted and deploy it on completely the wrong server. Some days it produces a report, other days it swallows 16GB and dies.

After he left we wrote the contents of his hard drive to this stack of CDs, and they are scratchy enough to have data loss. Of course the prod executable differs from all variants of code found on the CDs.

It turns out to be a custom reporting framework for producing exactly 1 report. For extra speed, it starts 100 threads but doesnt have any synchronisation. Which turns out not to matter as they are all fighting for 1 DB connection.

You are not allowed to touch prod directly, but have to get some unix guy on the phone so far to recover logs, executable, and config. And as that guy has to support this mess, he hates your guts by association.

I actually quite enjoy the true mysteries that nobody has touched for years. It's a situation where you're expected to slink off into a corner and swear quietly for a few days, and nearly-certain hero status if you pull it off.

The more frustrating situation it needing to jump in and make a one-off fix to something that is at least notionally "modern". These can be just as impenetrable, but you have to hold your tongue much more (especially if some of the developers are still around), and there's generally a lot less kudos for sorting things out. Personal "favourite": debugging stuff that's virtually impossible to build locally "...because we've got this great CD pipeline".