Backed over my own socket set. Luckily it was Made in Germany and in a steel box so it didn't really give a gently caress.
On more than one occasion swapped war stories instead of focusing and ended up putting the winter wheel I had just removed back on, instead of the summer wheel I had just switched to.
Sent my mate off to air up his spare wheels after we had cracked the lug nuts all around. Luckily he just went to the gas station down the road and at no point went above 30 kph.
Used a brake pad spreader with all my might and then realized that I left the reservoir cap on. The reservoir survived, but I sent brake fluid into the deep unknowns between the firewall and the front shock tower. Good thing paint is cool with that... oh wait.
Neglected to put a jack stand under the trailing arm rear suspension of a VW and then removing both coilover assemblies. It fell down and was caught by the brake lines and the ABS sensor wires. An ABS sensor went out shortly thereafter, and I think I know why.