The worst computer mistake I ever made was on my C64, way back when I had only one floppy drive. I wanted to make a backup copy of a disk, & the disk copying program I was using had to format the backup first. It was halfway done with the format phase when I realized I was holding the blank floppy in my hand and that the disk I wanted to copy was being formatted.
super-oops with computers
@Tobi I think I did one worse. I tried booting Windows 1.01 floppies on the family Tandy 1000, but Windows kept stopping, complaining about something DOS-related. I looked on the disk & decided that it wanted me to delete msdos.exe, completely ignorant of the fact it was the Windows shell (MS-DOS Executive) & not a bad or rogue DOS kernel.