Oh jesus, where do I start?
Removed a knackered old alarm from a car, didn't check it over properly which meant a couple of bare wires shorted and it caught fire.
Trying to get the back bumper off a MGB, which required a ring spanner and a lump hammer. One big belt and the spanner flew off and through the main bay window of the house.
Forgotten to tighten wheel nuts so many times it's silly.
Did an oil change on my Saab and didn't put the filler cap back on. The engine bay was given the best rust protection via a coat of Duckhams you could imagine.
Replaced a worn out piece of hose used on the fuel line on my SD1, started it up and saw this fountain shooting up between the vee. yes, I'd managed to find a bit of hose that was
more perished than the one I'd thrown away.
Didn't watch was I was dong and used WD-40 instead of brake cleaner to wipe down some discs. Wheeeeeeeeeeeeeeeeeee!
Took the wheel off a Rolls Silver Spirit using the car's jack to look at a bulge in the inside of the tyre. Car fell off jack.
Replaced the mirror casings on my old car as the originals were scratched for shiny new painted ones. Jumped in, flipped the drivers side one out, reversed back down the drive and heard the drainpipe on the side of the house rip the entire mirror assembly off.
I'm sure I can think of a few more!