Just about everyone in Washington seems agreed that Rep. Joe Wilson, R-S.C., committed a terrible gaffe the other evening when he shouted 'You lie!' during a pause in President Obama's address to Congress on Wednesday. I am not certain that he did, but this episode does nicely illustrate Michael Kinsley's oft-quoted maxim--which is not likely to be invoked much this week--that the definition of a 'gaffe' is when a politician inadvertently speaks the truth.