Andrew Stanton pitched his idea and story to Pixar head John Lasseter in an hour-long session, using elaborate visual aids and character voices. At the end of it, an exhausted Stanton asked Lasseter what he thought, to which Lasseter replied, "You had me at 'fish.'"

Director Andrew Stanton did the voice of Crush, the sea turtle. Stanton never intended to do the voice of Crush, only providing the voice for the film's rough cut, but when it proved popular in test screenings, he decided to do it for the final film. Stanton recorded all of Crush's dialogue lying on his couch in his office.

This movie surpassed The Lion King (1994) to become the highest-grossing animated film. The Lion King (1994) producer Don Hahn called director Andrew Stanton to congratulate him and said, "It's about time."

Pixar developed a realistic look of the surface water but had to make it look fake so people wouldn't think it was actual footage of the ocean surface.

Rendering a frame (at a resolution of 1600x900), which lasted about 1/24th of a second, in the film, could take up to four days because of the complexity of the underwater environment, with sunlight coming through the water and hitting fish scales.

Continuity

When the dentist is scooping Nemo out of the aquarium with the bag, the first time the bag is seen, it has a zipper top. The next time the bag is seen outside the aquarium, it is a plain bag with no zipper top.

When Bruce is chasing Marlin and Dory, the mask in his mouth switches from the right side to the left side and then back again.

When Nemo is captured by the divers, the boat has an outboard engine on the back. When the boat leaves it is seen to be travelling with no engine on the back.

When Nemo first encounters the diver's boat, the anchor is deployed. When the divers return to the boat with Nemo, they start the engines and leave immediately without retrieving the anchor.

When Marlin is telling his story to the small turtles in the East Australian Current, their shells look different than in other scenes.



Factual errors

Marlin and Dory are advised to swim to the back of the whale's throat and to be blown out of the whale's blowhole. This is not physically possible as whales' mouths and digestive systems are not connected to their respiratory system and blowhole.

Nigel the pelican has nostrils. Pelicans do not have nostrils. They breathe through their mouth.

Coral should be bigger than Marlin. Female clownfish are always bigger than their mates (often twice as big).

Chuckles, the fish that served as Darla's present the previous year, is a goldfish, who live only in freshwater. The dentist's other fish are all saltwater fish - thus, Chuckles should not have been able to survive in the tank with them.

Bloat and Jacques shouldn't be in the same tank because invertebrates are the porcupine fish's main food.



Incorrectly regarded as goofs

The divers' boat has a stern-drive unit with two propellers. When it starts, both propellers turn in the same direction, but they should rotate in opposite directions. Actually, the second propeller was seen from the other side, so they were rotating in opposite directions.

When Dory and Marlin are hanging onto the whale's tongue, they look down the throat and see the whale's uvula. Humans are the only animals with uvulas. Director Andrew Stanton and crew gave the whale a uvula so that audiences would understand that Marlin and Dory were inside the whale's mouth.

Sherman is shown performing a root canal towards one patient and later pulled a tooth out of another patient's mouth. Both patients are shown reacting to the pain. In reality, whenever a dentist performs these tasks, they often inject the shot inside the gums so that the patient doesn't feel any pain during the whole process. However, this is probably done for comedic effect, showing that the dentist is not good at his job.

When Mr. Ray sings his "Let's Name the Species" song, he isn't actually naming species. Porifera, Coelenterata, Ctenophora, Bryozoa, Arthropoda, Echinodermata, and Chordata are all phyla. Hydrozoa, Scyphozoa, and Anthozoa are classes in the phylum Cnidaria (Coelenterata), and Gastropoda is a class in the phylum Mollusca. The order of classification is: kingdom, phylum, class, order, family, genus, species. Phyla and classes are definitely not the same as species, although it is revealed, in the DVD, that Mr. Ray isn't an actual scientist, more that he just picks things up. In that case, it could very well be that he mistook the one for the other, not knowing that he was passing on incorrect information.

With Clownfish, the population is mostly female. One of the bigger females will change sex to become a male when the male disappears. As the big male is still there (Nemo's father), and Nemo isn't in adult life yet, Nemo should still be a female. However, the filmmakers deliberately decided for Nemo to be a male because a male changing to a female would be considered too suggestive for a family-oriented film.



Revealing mistakes

When Nigel drops of Marlin and Dory by the treatment plant there are several drop effects in the water as if it's raining, yet no rain is visible.

When Dory is comforting Marlin in the whale, the sequence earlier in the movie when Dory meets Marlin for the first time is reused.

The fullscreen version of the film accidentally reveals the visible portion of a woman's legs as she enters the waiting room, essentially showing some of her bare leg as the hem of her skirt is now seen.



Anachronisms

The sharks hold their meeting on what appears to be a World War II-era American submarine sunk by a mine strike. It makes no sense for an American submarine to have struck a mine in Australian waters, since during WWII the U.S. had three submarine bases in Australia and would have been fully aware of any mine fields near the coast.



Audio/visual unsynchronized

Peach has to pull away from the glass tank in order to speak clearly, as her voice is usually muffled. However, when Darla arrives, she speaks clearly without pulling away from the glass.

While attacking Marlin and Coral, the huge barracuda is roaring. Barracudas are fish and do not have the ability to roar. However, this might have been done intentionally in order to add dramatic overtones to the scene.



Errors in geography

Nigel is a brown pelican. The movie is set in Australia, where there are no brown pelicans. Instead there is the Australian pelican. Both birds look quite different.

The diving flag seen on the dentist's boat is the United States' "Diver Down" flag (red with white stripe). This flag is not used in Australia, and would not be seen on the Great Barrier Reef. The Australian version is blue and white.

The AquaScum 2003 measures the tank's temperature in Fahrenheit. Australia uses the metric system, so the tank's temperature should be measured in Celsius.

Nemo and Marlin both have American accents, but are Australian natives and would have Austrailian accents.

The barracuda that attacks Marlin and Coral appears to more so resemble a Great Barracuda, the world's most deadly barracuda species, which doesn't occur in the Great Barrier Reef.



Plot holes

Nigel saves Dory and Marlin from being eaten by seagulls. In order for Nigel to do that, he had to scoop them up into his mouth as well as water for them to breathe.

When Nemo gets flushed and is racing down the sewers, he emerges from below the surface of the water and takes a sharp breath as if he were gasping for air. At several other points in the film (when Marlin is chasing the boat and has to go to the surface to look, and when Nemo is about to leap into the filter of the tank) the animators took care to have the fish go under the water to gasp for breath.



Character error

Crush the sea turtle claims 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, which are land turtles, that are known for their very long lifespans and, even then, the current record is 188 years old, not much older (relatively speaking) than Crush.

When Nigel tells Nemo about that Marlin is looking for him, he mentions that Marlin has "traveled hundreds of miles" to find his son. In Australia, they use kilometers, not miles.

Many characters often claim that "All drains lead to the ocean", however there are endorheic basins were water never reaches the ocean, but flows away from the ocean and can only escape through evaporation, including a large one in Australia where the movie is set.

When Marlin is speaking with the three dads and Sheldon, Tad, and Pearl come to greet Nemo and comment on his "lucky fin", Tad says, "He looks funny!" and Sheldon's father smacks him and says "Be nice! It's his first day." However, it was Tad who made fun of Nemo, so Tad's father should've been the one to react.

When the dentist begins performing the Schilder technique while filling a root canal, Bloat and Gurgle get into an argument over whether the file he is using is a Hedstrom or a K-Flex, judging solely by its cross-section. However, the dentist is shown filing with the sawing action, more closely associated with Hedstrom files. K-type files are more rotational and drill-like. Therefore, Bloat would be correct, and Gurgle would not claim otherwise.