Had a few **** ups I can think of:
Had a car where there was a parking ticket in between the windscreen and the trim. Thought it would be easy to slide the ticket out with a knife or similar. Nearly got it... "crack". Much easier to take the ticket out whilst the windscreen is being replaced...
Friend put 16 cans of Stella in my first car in the boot. I forget and then remembered as I heard some 'pssst' sounds coming from the boot. Took a while to clean up...
Same car - lost it going up a hill and fishtailed until I finally hit the kerb at the top of the hill (luckily nothing coming the other way whilst fishtailing). Replaced the broken allow with the one from the boot and bought a steel wheel for the boot. Replaced the 2.0i back axle with one from an old 1.9 Manta.
When I sold this car - it had a flat battery. Left it on charge all night before the day of sale, and prayed while we went out in the rain with wipers, lights, radio on etc... that it would start when they finally left. It did...
When we last moved house, we moved with the 30 litre Bi-Orb 3/4 full in our car. On arrival, only a quarter of the water left... Hmm. Floor was a bit wet in that car...
First time I changed oil, I forgot to buy a new sump plug gasket. Tricky to get one without the car...
Replaced the front speakers in my previous Saab and wrecked one of the old ones taking it out accidentally. New speakers were shyte so I swapped the old ones back in (which turned out to be Pioneer!) but I couldn't use the wrecked one. So it now has one new speaker and one old speaker and I could certainly hear the difference between sides before I sold it.
Spent £200 doing all the rust on the sills, wheel arches and boot on an Escort. Then wrote it off a week later...