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.
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.
Revealing mistake: After young Josh is granted the Zoltar wish he drops the cable. In the following shot of him walking away not only it is not next to the machine, but it runs along the concrete, revealing it is plugged somewhere offscreen.
Continuity mistake: When Josh and Susan talk about the electronic magazine at Susan's place, the slice of pizza on the plate moves between shots.
Continuity mistake: When Josh and Susan arrive at Josh's apartment, the distance between Susan and the dinosaur changes twice between shots.
Continuity mistake: Josh stumbles into McMillan, scattering papers everywhere. Susan picks up hers and presses them against her chest. In a close-up they're tidy, yet a shot later they're all messed up.
Audio problem: When Josh is about to be interviewed, a woman says "Personnel director would like to see you now" but she's not mouthing the lines.
Continuity mistake: In the equipment room, after adult Josh sings to Billy, his hair swaps from messy to brushed right before he says "Josh you look terrible."
Continuity mistake: When Billy enters the equipment room and confronts adult Josh, note the top ball's yellow sticker: it's either facing his face or away, depending on the angle.
Continuity mistake: Before adult Josh leaves home, he grabs Zoltar's card by twisting his hand around. In the close-up, the hand's position is different.
Factual error: The closing credits give thanks to "FAO Schwartz", misspelling the name of the famous toy store with an extra "T".
Other mistake: When Josh and Billy are over the balcony dropping the water balloon on the mover on the street, the balloon is shown dropping from an overhead angle off course from hitting the man below, yet we hear him scream being hit, when it easily would have missed him where Josh dropped it.
Continuity mistake: While Billy and Josh read the newspaper ads, note Billy's ice cream: the spoon stands-out longer or shorter and the pattern of the chocolate differs between shots.
Answer: Paul (John Heard) tells Josh that the serve has to cross the yellow line. When Josh calls the ball short, Paul is basically trying to cheat and claims he never said the ball has to cross the line. Although I'm trying to figure out what racquet sport their playing. It looks like they're playing on a handball court, but the racquet sports I know, the ball has to cross the short line (the yellow line in "Big") on the serve but you also serve from a service area where you have to stand in front of the short line, which they don't do.
Bishop73