THe list is long and full of ballsups
1) first PC went pop, was fairly sure it was the MOBO swapped it, nothing happened, fiddled with everything for a while couldn't figure out was wrong, only thing I hadn't touched was a little red switch at the back of the PSU. so I flicked it

1 PSU on fire and copper raised from the mobo. I didn't bother trying to determining if anything that was connected to the PSU at the time was worth salvaging
2) ME (actually this should be number 1)
3) In the 12 years I have had my own PC I don't think I have had a single year where I have not had to reformat at least once, however in later years I have had more PCs to balls up so I think statistically I'm getting better.
4) HDD died (not my fault, it just decided one day it wanted to play brick) all my downloads, music and recorded TV and family photos were on that disk. No way to recover without paying

luckily the photos were backed up on several other disks. Still I'm 500GB of media down which will soon need to be replaced.
5) I think my dad is at fault for this one. We were trying to create a home NAS and some how through the constant on/off/on/off/on/off the computer just died, we thought the power switch may have failed but on further inspection we think something on the MOBO died.
6) Another one with my dad. My younger brother had spent a whole week trying to fix his computer, the HDD had died, and he had bought another one, but for some reason whenever he reinstalled XP the PC would just die and would fail to boot.
So we had a look at it, and for ages we could not figure out why it would not even read the XP DVD let alone boot a disk. Turns out there were 2 DVDRoms and the one we put the disk in did not have its SATA cable connected

Next we installed XP on the new disk and we had issue after issue, sometimes it would install sometimes it would crash, so we decided to upgrade the BIOS, which seemed to work.
So we then installed XP and spent a while downloading the drivers off the internet on another PC and transferring them onto this PC so that the networking would work and we could run the MS auto update and get the latest SP release. After all this we rebooted the PC and it came up with "OS missing". Oh crap... So we started again and this time we rebooted before installing anything on top of the OS and it was fine. Then we did the networking drivers and SP3... And again "Missing OS" when we rebooted.
Turns out that the boot order in BIOS was CD USB Floppy HDD and we had the USB dongle still hanging in the back

7) Bricked a router when flashing it once.