Not buying 3rd edition starter decks or dual lands because $10 was way too much to spend (hello five packs of Fallen Empires for $2 each).Literally throwing Ice Age / Alliances / Homelands / Fallen Empires cards off a balcony when I didn't want to bother dealing with my friends and they wouldn't go away.
Passing up $20 boxes of Fallen Empires and $40 boxes of Alliances at card shows.
Trading 4th Edition, Ice Age, Alliances, etc., rares to the local card shark for 3rd edition cards like Kird Ape and Counterspell because Revised was so much cooler. I think I had about 20+ Kird Apes at one point, but a distinct lack of Royal Assassins, Force of Wills and Icy Manipulators...
Having my best deck and half of the cards from the first World Championship box set get stolen from said 'friends' before I moved.
Selling my Mox Jet for $100 in high school and thinking I made out like a bandit because it paid for a weekend of debauchery.