Some of the most embarassing:
1) One time I was trying to boot from a Knoppix CD, but it didn’t work (despite the boot order being CD-first.) When I opened up the drive, I found out I was trying to boot up from a Tom Petty CD (Full Moon Fever, to be exact.)
2) One time at school, the monitor was stuck in power-save mode. I tried turning it on and off several times and checking the cable connections when the student beside me noted the computer wasn’t turned on.
3) One time I forgot to set the compression settings in VirtualDub and ended up with a 15-gig file for a 10-minute, low-quality video.
4) Tried to burn a CD before putting anything on it.
5) Wondered why my HDD was filling up. Realized I forgot to empty the Recycle Bin (which was filled with a bunch of DVD VOB files, which get huge.)
6) Accidentally killed my DVD drive. I saw it in Safely Remove Hardware and I removed it to see what would happen. Came back on reboot.