Continuity: Susan's hand on Josh's chin when he's about to return home.
Continuity: One of the racquetball players has his racket jump from hand to hand.
Continuity: Big Josh hands the baby his wallet while she is in the playpen, yet when he picks her up, it is no longer in the playpen with her.
Continuity: When Josh hands his wallet to his baby sister, she turns and walks to the center of her playpen. In the very next shot, she's leaning up against the railing again.
Audio/visual unsynchronized: When Josh and Paul are walking to the court, Paul says a line but his mouth clearly says something else.
Continuity: When Paul and Josh are at the court, and Paul keeps asking for the ball, Josh drops his racket in one shot, but it's back in his hand in the next.
Continuity: When Josh first meets Billy, after turning older, a dodge-ball on the floor by Josh appears / disappears between shots.
Continuity: When Josh and MacMillan are on the giant piano, the crowd behind them changes a lot between shots, e.g. 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.
Revealing mistakes: Josh plays his computer game. He replies to the question "What do you want to melt him with?" You can see him typing, but watch his right hand, he just hits the same keyboard key over and over again.
Continuity: The amount of wine in Susan's glass, before she goes to talk with Josh at the party.
Plot holes: The morning after Josh gets "big", he tries to convince his mom that he is really her son. He tells her that his birthday is November 3. Later, we see a milk carton with a "missing" advert on the side about Josh, which says his birthday is 1st January.
Continuity: When Young Josh is at the Zoltar machine, just before the machine starts working, the "man" in the machine has it mouth halfway open. In the next close-up, the mouth is now closed.
Continuity: In one scene, Billy is putting items in a bin, including his walkie talkie. Between shots, the items such as some pieces of paper, change position.
Continuity: When young Josh is standing in front of the Zoltar machine for the first time, Zoltar is opening and closing its mouth and its eyes are flashing red, but when we see it from the side, the head is still.
Continuity: When Susan chases after Josh after he runs out of the meeting, she just misses the elevator doors closing. She pushes the down button to call another elevator, but it was already lit - there's no reason for it to be lit when a descending elevator has just closed and left and nobody else is there to push the button.
Factual errors: In the closing credits where acknowledgments are given to entities who assisted in the production, the entry for "FAO Schwarz" is spelled "FAO Schwartz."
Revealing mistakes: Josh is obviously wearing a flesh-colored garment under his sweater when he and Paul have the fight at the racquetball court, evidenced by the wrinkles on his back when he first falls to the ground.
Audio/visual unsynchronized: Billy's mouth as he says, "...all the way down into her flowers."
Boom mic visible: When Josh and Susan are standing up through the sunroof of the limo after leaving the party, the shadow of a boom mic is visible several times on Josh as the limo passes under street lights.
Continuity: When Josh and Susan are riding in the limo, the background shots alternate between different streets, evidenced by the lights and buildings which are constantly changing between shots.
Audio/visual unsynchronized: When Josh and the St. James Hotel guy are talking about the $10 deposit for the sheets, the man's voice continues into the next shot without his mouth moving.
Audio/visual unsynchronized: When Josh's mom is freaking out when Old Josh walks in, she is speaking (screaming) but her mouth does not move.
Continuity: When "old Josh" meets Billy for the first time in the ballroom, the football helmets change position on the table beside Josh throughout the shots.
Continuity: A red sticker appears on Josh's door in the hotel which wasn't there earlier.
Boom mic visible: In the scene where Josh and Susan are jumping on the trampoline, a boom mic is reflected in the window.
Continuity: When he wakes up as big Josh, his underwear seems the right size, but if he wore them the night before, they should surely have been too small for him?
Audio/visual unsynchronized: At the Macmillan party, the upright bass player at one point stops playing and says something to another man, then carries on playing, but you could still here the bass even though he stopped playing.
Errors made by characters (possibly deliberate errors by the filmmakers): When Josh and his boss are playing piano at the toy store, Josh throws a bag down on the ground before playing. When Josh and his boss leave Josh leaves the bag on the ground and doesn't take it with him.
Continuity: When Josh and Susan are at the carnival, she has mustard on her lip. When she wipes it off, her hand is bare. Yet when we see her hands holding the stuffed animals, she is wearing her gloves on both hands.
Continuity: When Susan drives Josh home at the end, the trees on his street are in full autumn color, but in the next scene when Josh and Billy are walking down the street, the leaves are green again.
Continuity: When Josh is telling Susan about his electronic comic idea, the sleeve on her left shoulder keeps changing. One shot it's off the shoulder, then it's back on the shoulder, then it's off the shoulder again. This happens several times.
Continuity: When Josh is discussing his electronic comic idea with Susan, the slice of pizza in the middle of the table has its crust facing the camera in the first shot, then it moves 90 degrees around the plate.