Ferris: Life moves pretty fast. If you don't stop and look around once in a while, you could miss it.
Near the beginning of the movie there is a brief scene around the empty house. While the camera is in the kitchen you can see a reflection of one of the set guys moving in the fridge handle.
Charlie Sheen (the guy in the police station) stayed awake for 2 days, so he could get the right look for the scene.