In the movie "Abe Lincoln in Illinois" (1940) Lincoln is played by Raymond Massey (Remember that--it's important!). During the scene where Lincoln's train is about to pull out of the station, we hear a crowd of extras saying "Good-bye, Mr. Lincoln" and "Good Luck, Mr. Lincoln". During the shooting of this scene, one of the extras decided to toss one over the fourth wall and ad lib, confident that the crowd noise would drown him out. To his horror, there was a lull in the crowd noise, and he was heard, loud and clear, saying "Good-bye, Mr. Massey!"

You'd think that, after a gaffe like that, that extra would never work in show business again, but the mistake was never caught. He owned up to it after he'd become famous. He was Ted Knight, better known as bumbling anchorman Ted Baxter on the Mary Tyler Moore show.