i've done two major dumbass moves.
in the supra, i failed to plug in the walbro connector all the way, had to figure out why the hell the pump would work off and on until i dropped the tank and just *click* the connector was in all the way.
in my old POS audi i changed the headgasket and water pump, and like 50 other things and when i put the oil filter back on i failed to realize that the gasket had come off of the previous filter and stayed on the block. so when i put in the new filter no matter how tight it was the extra rubber ring that stayed on there just blew out 5 seconds after i first fired it up. My dad was crouching next to me looking under the car when the gasket blew and 3 quarts of oil sprinklered out.