Continuity mistake: When Tom Hanks is in the dinghy by himself, the outboard motor has no cover. He falls into the water and the dinghy circles him several times. One of the shots shows the motor with its cover in place. (It is an Evinrude motor).
Audio problem: After leaving Bloomingdales, Allen and Madison start talking. Allen says to her "Just begin when you were born and finish up right when you blew up the TV's." His mouth doesn't move at all when this is said.
Continuity mistake: When Allen and Madison are walking along, they pass a preztel stand, and you can see a woman walk past the stand and look at it. In the next shot there is nobody at the pretzel stand.
Continuity mistake: When Tom Hanks stabs Eugene Levy with a needle in the dentist's office, Levy is not wearing a bib. When Levy sits in the chair, he has a bib on. Then when he gets up, the bib is gone.
Revealing mistake: The saleslady in Bloomingdales tells Madison she shouldn't be wearing boxer shorts, however, when doing the exercises in front of the TVs, she bends over a few times and the lines show panties, not boxer short lines.
Revealing mistake: When Allen is in the park and drives backwards to avoid the MPs, there's a closeup where the pigeons fly backwards, revealing the angle is the previous one with the birds taking off but played backwards.
Continuity mistake: When the mermaid rescues Allen after the boating accident, the amount of sand on Allen's face changes from the whole side of his face to just his chin to hardly any sand at all. This goes on for the whole scene.
Continuity mistake: When Madison leaves Allen's apartment to go to Bloomingdales the tie she is wearing is plain navy blue, when she arrives at the store it has changed to blue with diagonal red stripes.