This is such an open ended question that I doubt the value of the thread, but here's my thought that's different from a few of the others.
It is only "OK" when the players are about to actually break the game. This is IME quite rare, but it is also not impossible.Spoiler: Obligatory War Games, the Movie, referenceSometimes, the only choice is not to play.
Beyond that, Phoenix's point on an ounce of prevention being worth a ton of cure is pretty good advice.