One of the most terrible ones I've ever made was letting an opponent trick me. I had built a BW Orzhov deck, which was a creation of mine, so no one was prepared for it. I was playing against Project X, and I had some graveyard hate (Planar Chaos wasn't out yet, so no Extirpate) against it, and was playing a tight game, but an army of THREE Saffi Eriksdotter pounded me to the ground, saving every relevant beater/combo piece when I tried for Wrath of God. After the game I realised, Saffi is LEGENDARY! Served me well, as I never again missed on proof-reading my opponent's cards.