You're asked a question--and you should know the answer. But you don't. Rather than fumble for some semicoherent response, maintain your cool and cite novelist Milan Kundera's observation, "The stupidity of people comes from having an answer for everything." That will at least get a good laugh, especially if you're in front of a large audience. It's a savvy way to admit that you don't know.
To take another awkward situation, say a scheming peer points out a glaring error you made (in front of your boss, no less!). Rather than defend yourself, give a clever riposte. Quote author Gertrude Stein: "Everybody gets so much information all day long that they lose their common sense."