When I first played FF7, I messed up royally with the scorpion robot you fight with Barret in the reactor. I read it as "fire when it's tail is up" and missed the "and it'll counter attack" as it was just a broken up window. Yeah, got my ass handed to me.
Part two of this was that I forgot how to get out of the reactor, and it blew up with me inside it. FAIL.
Also when I faced Sephiroth in KH2, and I thought I beat him but the screen didn't slow down as it does when you win a boss battle, and I put down my controller. He killed me right afterwards, the cheat!