Errors made by characters (possibly deliberate errors by the filmmakers): When George comes into the room when they are playing the piano he says, "Is that G stuck again?" when actually the stuck note was a C.
Continuity: When Stuart is in the sewer, he gets wet. When he climbs out of the drain, he is dry again.
Continuity: Snowbell loses his collar and tags into the lake when the Central Park altercation takes place, but he is wearing them again when he returns to the Littles' house.
Continuity: When the piano is being played, the note that gets stuck changes pitch depending on who plays it.
Continuity: At the time when Uncle Crenshaw asks for his favorite "Little" nephew at the family gathering, Grandpa Spencer is not in the group shot. After switching shots and going back to the group shot, Grandpa Spencer is there.
Continuity: In the scene when George throws his pajamas down the laundry shoot with Stuart tangled up in them the bottoms are already in the basket.
Continuity: When George and Stuart are asked to leave the room while the Littles and Stouts talk, George picks Stuart up in his palm and Stuart's tail hangs over the edge of George's hand. When they show the two walking out of the room, Stuart's tail is now underneath his butt and not hanging.