Not valuable by any stretch, but I had a chance to buy a Manta Montage for $1700 when I was 23 years old. It was in great condition, but I thought the money would be better spent on alcohol and blow.
Also, I bought a 1970 GTO convertible when I was 16. It had a seized 400ci in it. Paid $400. Tore the motor down, didn't want to spend the money on engine parts so I sold it for $100. What a loser!