In a scene about 50 minutes into the movie, a shot shows Ferris' empty house. They show a part in the kitchen and you can see a reflection in the refridgerator handle of someone on the set.
I don't disagree with you completly. This could have also been a car passing outside the window though, or perhaps a person walking a dog outside. It could have all been for effect. Or, like you have proposed, it could have been a slip-up on set, but the director/editor did not find it necessary to redo because it could be taken for many things.
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