Rules for the GM
Rule #3: The GM is (almost) always right. Player catch you in a mistake? Did your NPCs do something they shouldn't be able to do? Did the NPCs know something they shouldn't know? Were they two places at the same time? You could just admit you made a mistake or you could act like that was meant to happen. Act suprised and pleased that your clever players caught on to your subtle clues and make up something later to explain it. Depending on the setting and the scenario there are many ways to explain strange things that seem like GM mistakes at the time. In Amber and other settings with a lot of intrigue there are tons of conspiracy theories that could explain things. In games with lots of weird technology and special powers you can go really crazy with your ideas. You don't have to explain things right away. Just keep the players occupied and be careful not to clear up the strangeness right away. This leaves the player with a mystery and gives you the chance to figure out what really happened. When you figure it out, let the players start uncovering the secrets behind the mystery.