What happens when you call the ball out and it is not? The April 2009 issue of Tennis Magazine has a really interesting rule explanation with a very fine line between what is right and wrong.
The question was basically: My opponent called my shot out; but when I questioned the call, he looked at the mark and said it was in. But he said we should replay the point because he had put the next shot back in play.
The magazines answer is interesting: If he had immediately corrected his bad call, while putting the ball back in play, you would play a let (unless you had a sitter as the next shot). But since he changed his call only after being questioned, he loses the point (The Code, Item 16).
I did not know that!