My first was on a '68 camaro I had during high school. I wanted to put a new intake and carb on the 327 engine so I bought all the parts and went at it. I was 17 and I completely ignored anything anyone told me about torque and bolt patterns and after the first time installing everything it ran terrible, backfires, etc... Did it again and actually listened to advice and read the instructions got a torque wrench and things went great.

Next, decided to do the timing belt/water pump etc. on my '99 Audi A6. Taking the bumper off to get at the engine took a couple hours by itself. While pulling the cam sprockets to replace the seals that probably didn't even need to be replaced, one of the cam sprockets popped off and bashed into my radiator. I hope for the best and put everything back together and what do you know, coolant leaking on the ground. Had to take the bumper off and buy a new radiator.