Continuity mistake: After Dory and Marlin passed the jellyfish and Marlin passes out, when the movie returns to the tank, the camera pans over the tank from right to left, and there's a reflection of the Sydney skyline. However, neither walls nor window frames show in this reflection. (00:45:55)
Character mistake: In the scene where the dentist is doing a root canal, Bloat says "rubber dam and clamp installed?" and another fish replies "yes", but when the picture goes back to the dentist and patient you can plainly see that there is not a rubber dam on the patient. A rubber dam is a large piece of latex attached to a metal frame, to keep water from the dental handpiece from going into the patient's mouth. Although all of the dental jargon in the film is precisely accurate, this device would have been very noticeable.
Factual error: Crush the sea turtle says he's 150 years old and "still young." Actually, at 150, Crush would be almost twice the normal lifespan of a sea turtle, which is about 80 years. Contrary to popular belief, not all kinds of turtles have lifespans of over 100 years. It is tortoises, not turtles, that are known for their very long lifespans and, even then, the current record is 188 years old, not much older than Crush.
Audio problem: When Marlin and Dory swim into the deepest part of the ocean to reclaim their mask, they see a light (which we know later, belongs to an angler fish). They stop right underneath the light and says "What is it?" but his mouth doesn't match the movements. It looks as though the animators had Albert Brooks (Marlin's voice actor) say a different line than what was animated.
Factual error: Clown fish are born as hermaphrodites and when a dominant female is found they become male. They only become female when their female mate is dead/gone. Then a hermaphrodite clown fish will become male and be the new female's mate and the process starts all over again. Coral should not have been the same size as Marlin; she should have been bigger, since clown fish that turn female will then grow bigger than their mate if they aren't already. (00:01:00)
Continuity mistake: Decorations inside the fish tank often disappear, appear in random places, or change directions and spots throughout the film.