I broke the first CPU I've bought for my own money, it was a PPro 180mhz back in early 1997 (2 days before Intel introduced MMX). I ran it overclocked at 200mhz for a good while until I started getting stability issues, had to downclock to 133mhz to use it after that (even 180 was unstable) and bought a new computer once I started my first "real" job.

Seen other HW issues, memory or motherboard(soldered memory) on my ex's laptop that affected only certain adress ranges, memtest86 has been my go-to to check computer health when random crap starts happening since then and I've replaced at least one memory stick on another machine thanks to it.