Factual error: Throughout the movie, to make it more authentic, small impurities (specks) are seen floating in the water. Throughout most of it, with great attention to detail, the impurities are always seen to be moving with the flow of the water - back and forth. This always works except in the first scene with Marlin and Coral. The sun is shining through the water, and you can see the anemone waving in the water flow, but the impurities are all flowing in one direction, left to right, never changing according to the water movement.
Factual error: After Nigel returns Marlin and Dory to the ocean it is raining and the sky is nearly solid with gray clouds. As Marlin and Dory speak bright sunlight is streaming down through the water all around them, illuminating the ocean floor. Even if the water is extremely shallow where they are swimming the overcast sky would not provide such bright, consistent light.
Factual error: At the end of the movie it shows all of the aquarium fish in the ocean inside of their bags. It also shows the bags almost completely out of the water with only a small portion under water while the bags are almost completely filled with water. With this amount of water the bags should be submerged underwater with only the little amount of air showing on the surface, because water is not lighter than water; water weighs the same.
Factual error: The film is explicitly set on the east coast of Australia. However Nigel is a Brown Pelican (Pelecanus occidentalis), a species only found in the Americas. The only pelican species found in Australia is the Australian Pelican (Pelecanus conspicillatus) which is mainly white, with black wing tips. If a Brown pelican was somehow transported to Australia and subsequently escaped into the wild, it would have been caught and destroyed under Australia's strict quarantine laws.
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.
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.