The biggest mistake You have made during working on your car?

[Vauhtiveikko]

As the topic says, what is the stupidest/worst thing that has happend to you when working on your car?

I was going to change my fuel filter this evening and when I reversed the car up for ramps I hit my garage sink with my newly painted rear bumper I left the car in the garage and opened some alcohol drinks to ease my pain

[spink]

leaving the 'diamond' washer out when doing a cambelt,that was upsetting

[Error404]

poured expensive new oil in before putting the drain plug back in the sump

[Acidburn]

letting someone near a engine with a dremel

[s2_wot]

letting someone near a engine with a dremel

MW special?

EDIT:
I should have said, I reversed my car one day with the drivers door open and kinda changed it's shape. That car is long since deceased. I think the bodywork type mishaps are the worst.

[irishs2]

letting someone near a engine with a dremel

Badum-Pish!

for me it was closing the bonnet, with the headlights fitted, but the brackets were loose around the headlight. Took nearly 3 hours to get the bonnet open again.

[Twenny Benson]

Forgetting to tighten the oil pump down on a freshly built 1.8T it lasted seconds

[Acidburn]

Forgetting to tighten the oil pump down on a freshly built 1.8T it lasted seconds

setting a car on fire with a plasma cutter

[Vauhtiveikko]

Thanks guys, helps I little that accidents may happend when you least expect them. I have to be happy that the sink bend so damage seems to be only in the bumper, not on the bodywork. Maybe RS2 style rear bumper and trim may cure my sorrow

[dspl1236]

building my v8q 5 speed convert

my buddy was welding but i was watching the reflection off the windshield...needless to say a few hours later bad uv burn...my eyes are watering typing this. i wore sun glasses for 3-4 days in any kind of light. ouch

[Twenny Benson]

setting a car on fire with a plasma cutter

Oh yeah I forgot about that!

[Twenny Benson]

building my v8q 5 speed convert

my buddy was welding but i was watching the reflection off the windshield...needless to say a few hours later bad uv burn...my eyes are watering typing this. i wore sun glasses for 3-4 days in any kind of light. ouch

Arc eyes for 3 or 4 days! man up

[rs2315]

No harm leaving tailgate wide open on your RS2 in a locked garage, or so you might think.
Problem comes when you leave garage by side door, and next day decide to enter garage by up and over door. BANG

and I've done it more than once

rs2315.

[DJones]

Drove straight off the back of a set of ramps forgetting i had already removed the calipers, imagine my surprise when i hit the brake peddle...

[Sleeper_Avant]

one of the muppets that worked for me put the exhaust cam in 180deg out after rebuilding a megane 16v head.......... the end result wasnt pretty.

as previousley said,
one of my first jobs was doing oil change in a e30 325is, i went through 12ltrs of oil before i noticed it had filled the floor up