Con the Commodore PET you could kill the computer with the “Killer-Poke”. On some batches of it, the command “POKE 59458,62” fried the video chip or was it the CRT’s electronics?
On the C64, while learning to program the 1541 disk drive, which had it’s own CPU, I managed to get it’s RW head “stuck”. I passed a wrong variable to it, the head moved beyond normal range and I had to open the drive and manually move the head back into the proper range.