Funny stories!
I once tried del *.* in dos thinking the A:\ drive was active. It was actually C:\ and I lost all the accounts for the department. I had to go through file by file with an undelete tool, guessing what the first letter of the file was. Stayed in the office repairing it like that all night. No-one ever knew.
Bought a new computer case (ATX, I think) and tried to fit it into an old style 486 case. The power cables didn't fit so I pulled the wires out and guessed where they should go and stuffed it all back in. (Don't try this at home).
All the lights in the house went out and I had to replace some fuses. The motherboard smelled a bit funny after that (burnt plastic) and surprise, surprise, it didn't work again.