I’m going by memory here, but I recall in the movie "The Verdict" (Paul Newman plays a downtrodden lawyer suing a hospital), someone says something like, "Never ask a witness a question that you don’t know the answer to." Of course at the end of the movie, the opposing lawyer (a smug James Mason) is thrown off base and makes the fatal mistake of asking a witness a question to which he doesn’t know the answer, and the witness’ shocking reply wins the case for Newman’s character.
But the expression on Mason’s face couldn’t match McCain’s in last night’s debate, when McCain gets a response from Obama to a question to which McCain didn’t know the answer (or didn’t know that Obama had an answer). Check it out, and watch McCain’s face (courtesy Talking Points Memo):