Continuity mistake: When Tom Hanks and the kid are in the ice cream bar, Tom asks the boy if he could have his cherry from the ice cream portion. When he takes the cherry from the boy's glass he drops some ice cream on his jacket, but the next time they show the boy, the ice cream from his jacket had disappeared. (00:28:15)
Revealing mistake: The opening scene shows Josh and Billy as kids walking down their street. At the end of the film, Josh is a kid again and the two are walking on the same street. The same cars are parked there, and the same woman is crossing the street at the same place. It's obvious both scenes were shot the same day. (00:03:20 - 02:06:30)
Continuity mistake: When Tom is dancing/playing on the piano keyboard, near the end of the song an overhead shot shows a woman and a child's feet approaching from the back, walking behind Tom to stand and watch. When they cut back to Tom from the front they are not there. Then they arrive after around 5-10 seconds later while the camera is still on Tom from the front. (00:49:40)
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.
Factual error: The closing credits give thanks to "FAO Schwartz", misspelling the name of the famous toy store with an extra "T".