Stan, sitting at the bar at the Blue Moon tavern, has downed several beers already and they've filtered through. His bladder is about to burst and he has to make tracks to the men's room, but he's just had a fresh pint delivered. He knows full well that someone will guzzle it while he's gone and he'll come back to an empty glass. So he grabs a napkin, hastily scribbles on it "I spit in this beer," puts it by the glass, and makes a dash for the plumbing.
When he gets back, he sees that under his note on the napkin, somone has written, "So did I."