Claiming to have seen a mouse
When in a meeting, take a quick glance around the room, and then suddenly exclaim, "Oh no, I just saw a mouse! I'm going to go catch it."
This will usually get you excused, but be prepared to be called out on it later. It's a delicate art, but with practice, anyone can master it.
For more advanced techniques, visit our Claiming to have been called away page.