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: 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.
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: 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.