Biggest would be not buying a 1969 Impala SS with #s matching drive train, 427/425HP 4-speed Muncie. Great condition, except the underside because it had been under cover (not sides) in Maine for 20+ years. Wanted $11k, I didn't want to pay more than $10k............dumbA$$......the engine was worth that.
On the tool related side, I didn't buy an 8-9ft. tall "Alien Head" Canedy-Otto DP from the 30s/40s......It got scrapped.