An actor had been out of work for years because he always forgot his lines. One day he got a phone call from a director who wanted him for an important part in a play. All the actor had to say was, “Hark! I hear the cannon roar!"
Opening night arrived, and while he waited in the wings, the actor muttered to himself, “Hark! I hear the cannon roar! Hark! I hear the cannon roar!" The time for the entrance finally came. As the actor made his appearance onstage, he heard a loud BOOOOM! He turned around and said, “What the bloody hell was that?”
It reminds me of an occasion many years ago in an AmDram production I was involved in, when a very shy lady, playing a maid, had to come through the door on stage and say to her master that his carriage was waiting.
It was timed just as he was giving the heroine a passionate kiss goodbye. It was so convincing that the 'maid' blushed and said, "Oh! I'm terribly sorry." and backed out. Luckily the hero covered it well.