-Killing my opponent's ally before attacking with Champion of Irum
-Using Shadow Energy before attacking with Aldmor Interceptor (who now has 1 less ATK point)
-Using fire attacks/abilities against Furrion Terror or electric attacks/abilities against Entangled Wisp
-Getting excited that I drew the exact game-changing card I needed, then sac'ing it into the Resource pile instead of casting it b/c I forgot to hit 'Skip' during the sacrifice phase

What other gameplay mistakes do you make that we all could learn from?