Deliberate floppy errors
Back when I was in secondary school, the computer labs were set up with 4 PCs in the middle of each of several large square tables. CRT monitors sat on top of the desktop style cases. We (bored students) would swap all the monitor, keyboard and mouse connections between opposite PCs. The computers would boot up fine and appear to work fine, but users would get error messages when they inserted the floppy and tried to read something, or discover files that they didn't recognise if the person opposite happened to insert a disk at the same time. It caused hours of fun and frustration.