Ok this is actually of another mechanic I once worked with a couple of years ago.
He was doing a normal service on a Magna. After draining the oil and doing the filter he forgot to fill it back up with new oil. Another part of the job was to carry out a throttle body service, so he started the car up and was standing there spraying throttle body cleaner into the TB while practically redlining the engine. After about 5 minutes and a few questions from other mechanics asking if he had oil in it(coz it sounded like a tractor) he decided to check the oil and realised there was none in it. Now the funny thing is he put oil in it and put it out the front for the customer to pick up. Service manager checked the car and the motor was basically destroyed. Had to rebuild the motor for free. As you can imagine the mechanic lost his job.
I've only had a couple that I can remember I did. Done 4 tyres on a car once and somehow it came back with 3 blowouts
: apparently from the way i fitted them. I also left a spanner on a brake caliper. The customer came back the next day coz they had no brake pedal. The spanner had undone the bleeder.