so you've written yourself into a corner

It really depends on the issue at hand. Sometimes you DO need to just admit you made a mistake and turn back time a bit. This works on things like spells that were cast and shouldn't have been capable of being cast. In that case, it depends on how long the spell has been in effect. If its been in effect for too long, then it's probably okay to just shrug and end the spell there. Worse things happen. If the result of such a mistake is critical, and a character death is on the horizon, then it's okay to handwave the encounter as a victory for the party. Do better next time.

On the other hand, if you run into a serious plot hole, like a villain learning something impossible, or something not making sense, and the players call you out, a really good way to get out of it is to agree with the players. Just say how strange it is...then your players think you've just played them, and you get some time to contrive an answer.

Quote in context because why not: "When in doubt have a man come through a door with a gun in his hand. This could get to be pretty silly but somehow it didn’t seem to matter. A writer who is afraid to over-reach himself is as useless as a general who is afraid to be wrong." -Raymond Chandler.

My first campaign now runs for 3 Years. My players have reached Lvl20 (in DnD 3.5) I could continue to throw monsters at them but realy wont to start a new world, avoiding al the mistrakes I made and use what I have learned in this 3Years. But they want to continue playing their characters and see how far they can push them. How do I wing highlevel play without falling into the "minion of the week"?

ORCS ATTACK!!!!