I used to work in a role that was basically tech support for engineers. Folks would call, we'd look at their code and see where things were going wrong.
One customer calls in, they're having timing problems with a satellite control system.
I dig down through their code, and buried in one of the modules is a comment to the effect of:
"Once we upgrade to Windows 98, we'll need to change this call to the precision counter"

They never did.

This system was running XP.
Somehow, they'd avoided destroying satellites despite having the code run on Win98, and ME without fixing that call. It wasn't until they upgraded to a multi core system and XP that their gyros stopped responding correctly.

Holy shit.