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.
Other mistake: Josh's birthday, according to the milk carton, is in the winter. But, some time after he celebrates his birthday at the Italian restaurant, he returns to his childhood neighborhood and it's fall (colored leaves, etc.). It can't be the following year because he's only gone for a total of a few weeks.
Continuity mistake: When Tom Hanks is playing the piano in the toy store you see a man in the crowd lift his daughter onto his shoulders 3 times.
Continuity mistake: When Tom Hanks and his boss are playing the oversized piano, a young girl in the background is seen to sit on her dad's lap twice. Also, the boy in the sweater disappears, then reappears in time to applaud at the end of the song.
Continuity mistake: When Elizabeth Perkins sleeps over at Josh's loft, the pillows behind her head are flat and in the middle at the head of the bed. When they cut back from Josh, they are pushed up in the corner of the bed.
Factual error: The closing credits give thanks to "FAO Schwartz", misspelling the name of the famous toy store with an extra "T".
Continuity mistake: When the camera first pans up to Josh on the top bunk as "big," note his coat on the bottom bunk: The letters on the back are pushed together and three are barely visible. Yet when he returns from the bathroom moments later the coat is noticeably more spread out and many more letters are visible.