Deliberate mistake: Old Weird Harold's pants fall down twice in the film. His boxers are different colors each time; once they're red, the other time they're blue. The director admits to this deliberate mistake in the commentary, siting his interest in seeing if people will notice.
When Albert and his friends are in front of Doris' house they are singing a song about going to the park. As they are finishing the song and leaving to go to the park, you can see the reflection of the microphone in the window of Doris' house.