What's the dumbest mistake you ever made on your ride and what did it cost you?

Over the winter one year put a 544 lift cam in my 396 in a 33 ford coupe.Man that spring she was thumpin running sweet shifting at 6800.Coming home from swap meet out in the country several of us.Down shifted to creep across some railroad tracks long straight stretch of good road what else can I say I nailed it she was talkin to me till I floated the valves peened the lifter with the push rod locked the lifter ground lobe off the cam.Spent half the summer with the motor out of it.