Wow .... so many and so little time
Helping a neighbor get his old car started that had stood for years. Prior to starting it, and at my suggestion, we took the plugs out and filled the cylinders with engine oil to let it soak down past the piston etc. Thought it would help lubricate the piston and bores. Left it an hour or so and then turned it over .... as the pistons came up to TDC they shot jets of oil out of the plug holes .... it shot oil up the side of the house and hit the gutters. The wall is still stained now
As a kid of 19, I fitted a re-con short block to my MKIV Cortina. Would it start after wards, would it F**K. Ended up towing it behind my mates car for about 2 mile trying to bump start it ... no joy. Back at the garage some smart ass suggested we connect the fuel line to the fuel pump

started first touch.
20 years ago my mate had a Yam RD250 (aircooled) and kept it out side. He got fed up with the front discs going rusty in winter so he smeared them with a thin layer of grease

then asked me a week later why I thought his front brakes were ****

I was gob smacked he lasted a week of commuting on the bike with no front brakes.
[Edited on 4-3-2006 by Carbon749]