Dumbest thing I ever did:
I once sold a olds 442 convertible that I found out a month later had a special W-30 package on it that only about 500 had been built with.
I made a little money on the car but the guy who bought sold it a year later for 10x what he paid and nowdays that car can be worth upwards of $1.5-2.0 million
It still bugs the crap out of me.
Most embarrassing thing and close to life ending was my wife pulling a pair of chicks underwear out of my back pocket.
(thankfully a buddy bailed me out and called it a gag)