I've been messing with my XP machine lately, dealing with some drive issues (see link, above). Things were pretty much sorted out, but the machine's been running hot and a fan is noisy, so I did some quick fan swaps. In the process an IDE cable seemed to loosen from one drive, so I pushed it back in.
Then, a few minutes later, I noticed it was popping out a bit again. Darn thing -- it seemed looser than it ought to be. I pushed it in, a little harder.
With airflow reconfigured and fans in place I rebooted... Of course I got the friendly message telling me I needed a boot drive. Darn cable must have popped out. I popped the cover off and looked -- yes, it seemed loose. I went to push it in again, which was when my frontal lobes engaged and I began to sweat. I very carefully and gently pulled the IDE cable out.
I'd flattened one of the IDE pins. Which is when I remembered my last backup for this machine was 5 days old (due to a backup reconfiguration process) and was offsite. If the pin couldn't be fixed I was in trouble.
I turned the cooling fan on me and I very carefully used a variety of old surgical instruments to more or less straighten the pin. It didn't break and I was able to reinsert the cable and restart.
Lesson learned. The next time I pop the case for a non-emergent repair, I'll make sure I have a current backup in place.