Continuity mistake: In the scene where Tom Hanks goes back to his mother's house when he's "BIG", he tries to convince his mother it's him by telling her his birthday: November 3rd. However, in the scene (with John Heard and Elizabeth Perkins) showing the milk carton of Tom Hanks "missing" as a young boy, his d.o.b. is shown as 1/20/75.
Trivia: The big piano scene was actually rehearsed months before shooting. Tom and Robert had to practise on a cardboard cut out with exact dimensions of the real piano. On the day of shooting there were stunt doubles instead of them to film their feet. But once the director saw that they could do it, the stunt doubles were never used.