Warranty work: In the late 90's I was repairing a beige desktop Mac (early PPC), I needed to remove the logic board, and while attempting to pry up the logic board I slipped with the screwdriver, which ripped off a resistor in the process. As it was warranty work on behalf of the manufacturer (I was working for a service agent), all parties agreed it was a mistake that could have happened to any technician, so it continued to be covered.
Destroyed keyboard: I once spilt a Fanta on a white Apple keyboard, the clear plastic base with the full height keys, the last of it's kind before the current flat aluminium keyboards cam in.
Almost lost data: I was click happy once during the process of backing up a laptop for a staff member (planning to upgrade the OS), and instead if hitting backup, I hit erase. I was able to restore the data thanks to hard drive erasing only modifying the first block or two on the disk, instead of going to the time and trouble of erasing the entire disk.