I'm usually careful, however not long ago I got a FIC VT-501 socket 7 motherboard for USD 2 at a garage sale. It had a Pentium-133, 32 MB of RAM and was working perfectly. Only, the fan attached to the heatsink was rattling horribly, so I decided to replace it and picked up a socket 7 cooler from my stash.
The new cooler had a horribly tight retention clip, which also was quite sharp. I stupidly pushed as hard as I could to lock the clip in place and it scratched heavily the motherboard near the socket. Looks like one or more traces were cut since all that motherboard does now is "beep-beep-beep-beep-beep", and I can't get to the damaged section to try and fix it as it is under the little tab of the socket. 😒
Another one: a couple of years ago, I was cleaning my K8N-SLI motherboard. When I was going to remove the heatsink the thermal paste had dried up and it didn't budge. I pulled a bit too hard and the heatsink came right off... along with the CPU. I was surprised how easily the CPU came off with the lever down, and the pins were bent in all possible directions. Luckily the motherboard survived without a hitch. 😊