My younger brother typed format c: instead of format a: once on my Dad’s laptop that he was borrowing to do homework - he was up all night reinstalling Windows 95 from 26 floppys.
My older brother, when learning assembly, needed a memory address to write to on the hard disk of my Dad's business computer. Sadly he chose the first one as he hadn't discovered the File Allocation Table at that point. My Dad wasn't best pleased, even though technically the data was still on the disk!