I think it happens to the best of us. I'm not the best of us, so I get to goof a bit more often. The specific example I'm going to mention was using Dresden Files (a derivative of FATE for those who don't know) but I think this could happen to anyone.
So it's the second session of our new Dresden Files game. It's me (The GM who's DMed a few scenarios before and played a bunch with a different group, but everyone in that group knew the game) and four players, two of whom have read the books, one that's watched the show. None of them have played this before.
Middle of combat. I remind one player that he can't tag the same aspect twice in a roll, but then combat seems to be going well. The bad guys dogpile one of the players, and everyone pushes all out to stop them before they can really wreck him. Just as they drive them off, the heavily foreshadowed cavalry arrives- for the bad guys. The wizard throws up a shield, and they all run. As they're running, he decides to cast a spell to boost his senses. Several turns later, he asks if the party is being followed, and I say he can't see them. He reminds me of his super senses spell, which I had forgotten, and I say nobody is in sight. Play continues, and he continues to make good use of the supersenses spell. We have a good old time.
It's not until a few days later, looking up a completely seperate rule, I notice that it specifically says that spells end the round after they're cast unless extra power is put into them. Probably because the ability to stack spells on yourself forever would be hugely broken.
He's going to want to do that again next session. Thing is, the senses isn't really that bad- it was cool, and like I said we all had a good time. But I know this guy, and I know that he has a knack (as well as perverse enjoyment of) breaking games. If he tries that with armor or shield spells, or stat boosts to something other that Alertness, this could go badly. As a DM, how do you correct a mistake that you let slide for almost a week before noticing it?