Bender is running through the halls singing at the top of his lungs. He runs past some glass doors, it is clearly nighttime outside although the movie takes place during the day.
I have always wondered why in the world people always made such a big deal about this blooper. How do they know that it wasn't a window to another room that was dark because it was not being used on a SATURDAY????
Because if it was a window into another room, if the room was small enough, the light from the hallway and the brighter movie lights would shine into this room. Sets for movie are lit very brightly. The movie is clearly shot some at night. This room should have been lit a little and it wasn't.