Continuity mistake: Before Allen jumps to chase the mermaid, the helicopter is above him and he looks up. His hair doesn't move despite the rotors above. Then in the following shots it finally does move.
Continuity mistake: When the army exits the lab to find the mermaid the pavement is wet and the sky is cloudy. The following aerial shot shows the pavement dry and the light sunny. The light keeps changing with no continuity throughout the scene: cloudy, sunny, dawn, dusk.
Other mistake: Joe Cirillo is credited as playing "Sergeant Munson", but his name badge in the movie reads "Edwards."
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.
Continuity mistake: When Walter soaks Madison she falls forward towards the curb. In the close-up a flowerpot appears, which disappears in the wide angle, where she lies 45ยบ to the left of the curb.
Continuity mistake: Hanks puts Madison inside the car where the solar roof is open. When he starts the car, half a second later, the roof is closed.
Continuity mistake: Allen drives through a spotless park, but, when the angle cuts, a flock of pigeons appears ahead.
Continuity mistake: While Allen is stuck in a traffic jam, several close-ups of Kornbluth show a yellow cab next to him. In the wide angles the car swaps to a dark coloured private vehicle.
Continuity mistake: Freddie leaves the Penthouse magazine on a counter, where a woman behind picks it up. In the next shot, she picks it up again.
Continuity mistake: When Hanks sneaks Madison out of the lab, his right arm is close to her chest, covered by a grey cloth. When the angle changes the arm is uncovered.
Continuity mistake: When Allen is brought home blindfolded and is interviewed by the press, two journalists without a camera approach from behind. In the front angle, a split second later, a TV camera appears out of nowhere.
Continuity mistake: When Allen is in the water tank, before Madison is put inside, his fringe keeps changing between shots, despite him standing still.
Continuity mistake: When escaping from the MPS, Allen leaves Kornbluth alone on the street. The passers-by behind the car are inconsistent depending on the shot.
Continuity mistake: Kornbluth steps off the car to distract the MPs. When he kisses Madison goodbye you can see the wall behind has no basement entrance. When the MPs are about to run him over he jumps away and goes through a door that has suddenly appeared.
Continuity mistake: At the beach, Walter and one of the twins carry a box and are inches away from the boat. Half a second later, when the angle cuts to Allen asking the other twin, the first couple is standing behind, far from the boat, holding the same box and repeating the previous movements.
Continuity mistake: When Allen has dinner with Madison, his glasses of wine and water are far or close to each other, depending on the angle.
Continuity mistake: Freddie crashes his car against some wooden boxes. The way they are left changes in a matter of seconds with no one touching them.
Continuity mistake: When Allen first sees Madison on the beach, her hair is nicely crimped. But when she steps forward and kisses Allen, her hair is completely straight.
Factual error: After Allen and Madison jump into the water at the end of the movie and fend off the frogmen, they immediately come upon a coral reef and other aquatic features that appear tropical and which would not be found in the Hudson River/New York Harbor.
