i have been working and accidently cut a servos's wires.
accidently shorted some batteries
once i tried to oil a brushed motor, but used the wrong type of oil. sucker started smoking really bad after a couple of minutes!
i have put a 8x4 prop on a brushless inrunner that was meant to push a EDF. fried that motor in about 3 minutes!
im sure there is more things. screws that never got put back....screws that ended up in a gear mesh