TRUE STORY: the first 2 booster packs of pkmn TCG I ever got had a Charizard and a Chansey in, I got so excited I dropped them on the floor of the shop. This kid came over and offered to help pick them up, being young and naive I was gratefull. When I got out of the shop I realised that the 2 rare cards were missing, I told my Grandma, who was with me at the time, and she stormed off and had a shouting match with the kid's dad, eventually forcing him to hand over the cards. It was so embarassing.
Later, I traded the Charizard - the most powerful card at the time for a Rhydon, and cried when I realised what I'd done.
I also gave away an ancient mew (which i traded for like half my collection for in secret because pkmn cards were banned at my school at the time) because someone told me promos were fakes. Then I looked on ebay years later and saw them selling for £20, which would have been alot to me when I originally had it.