Although it may seem contrary to my post in the thread, "Do you know you're a bad GM", I will happily admit to making mistakes all the time. At least once a session, though I have been known to go two or three with not but a couple of minor errors. I've also bundled things quite splendidly on a number of occasions.
Actually I embrace my mistakes - I just love'em! I've gotten some of my best and most creative ideas from my forgetting a rule or creating a minor plot inconsistency. My players never know if I planned it or not!
GM: "And so with no where else to turn, the last Wizard of Zozrath teleports from the chamber in a flash of..."
Player 1: "What?! You said the Wizards of Zozrath can't teleport. It's against their code or something."
Player 2: "I have it here in my notes from our 5th session months ago. The arcane spirit of Zozrath refuses to allow his followers to learn the magic of Teleportation because it defies his personal code of honor."
GM: <Shifts his eyes from player to player and then leans back in his chair and smiles> "So the Wizards of Zozrath don't 'port. But this one did."
Players: <Angry> Yeah! <Inquisitive> "Yeeaahh." <Stunned and excited> "Yeah! Oh man...that guy was a fake. There's still one left out there somewhere!"
GM: <Cool outside...Laughing internally> "Exactly."
AD
"Never Give Up, Never Surrender!"