Character mistake: When Stuart calls home from the pay phone he's talking into the wrong end of the phone.
Continuity mistake: When Stuart is sliding down the drain he gets his hands all slimy when he tries to stop himself from going all the way down. When he is pulled out of the drain, he is clean.
Continuity mistake: When the little family is in the taxi chasing Stuart's airplane, the baby is awake in her car seat, asleep a second later, then awake.
Revealing mistake: When Stuart's dad is giving final instructions to Stuart as he's about to lower him down the drain, Stuart (and his bright yellow shirt) aren't reflected in the dad's glasses.
Character mistake: In the football scene, when the kid boots the ball to George who is upfield, he then hits it at the other kid, who is also on his team. There are two problems here: 1) The original pass would have been offside, as you see the all of the other players are still charging up the pitch. 2) When George kicks the ball at the other boy's face, that would have been offside as well, for reasons that are stated above.
Other mistake: When Margalo lands in Stuart's car after escaping the Falcon, Stuart slams on the brakes, only he hits the gas pedal instead of the brake.
Continuity mistake: When Snowball catches and eats the fly his collar is completely visible. In the next shot when Snowball see's Margalo for the first time only the bell on his collar is visible.
Continuity mistake: When Stuart is skateboarding in the birdbath the surface is smooth. Later when Margalo is in the bath with water the surface is in ridges.
Other mistake: In the scene where George kicks the soccer ball with Stuart on it there are two problems. 1) If Stuart was on the front of the ball he would have gotten smashed when the ball bounced off the fat kid's head and 2) The goalie that tries to stop the ball looks like he has a yellow jersey which would mean he is blocking his own teammate's shot.
Factual error: The screeching call occasionally made by the Falcon is actually the sound of a Red-tailed hawk. Falcons do not make this sound.
Continuity mistake: At the start of the movie, Mrs Little scoops up baby Martha out of her cot and walks over to the bedroom window. She has the baby on her hip when she sits down at the window to see the birds but in the shot from outside the window, baby Martha is sitting on her mum's lap.
Continuity mistake: At the soccer game, when the boy on George's team gets nailed in the gut with the ball, it rolls off his body. Yet, when they return to the shot of the boy lying on the ground, he is holding the ball. Who gave it back to him?
Continuity mistake: When Mr. Little and Mrs. Little are cuddling each other after arguing about overprotecting Stuart, Mr. and Mrs. Little swap places between shots.
Continuity mistake: When Margalo is going out for "fresh air", her wing is still strapped with her scarf. As soon as she takes flight, Margalo's scarf is neat around her neck. I know that her wing isn't realy broken, but there was no time to untie it, unstrap it, put it around her neck and tie it, all while flying.