This is a mistake for the introduction from seasons 2-20 (1991-2009). When Homer screams, he turns round. You see this in a wide shot. There are no boxes to the right of the door in the garage. However, two just appear out of thin air when he runs through the garage.
The Simpsons holds the record for most guest stars; it is also the only non-variety show to have had appearances from three former Beatles (Paul, George and Ringo).