Someone (usually a guy) wakes up in bed, and the audience assumes he's alone. But then either he moves or the camera pans, and we see there's someone else. Usually, it's implied that hanky-panky has just taken place, and if it hasn't, the guy will either anxiously ask for confirmation, or just assume the worst with hilarious consequences. A common variation is to have the guy wake up to find a another guy in his bed. In that case, either he's really anxious to ask for confirmation about why he's there, or he's too afraid to ask. Harem Anime loves this to death.