Continuity mistake: When Melody is in her room getting ready for her party, at one shot her hair is down. In the next shot of her, it's in a ponytail, and in the next it's down again.
Continuity mistake: When Morgana traps melody in the cavern, Flounder is seen in his younger size. The next time Flounder is seen he is back in his adult size.
Continuity mistake: When Morgana sinks Prince Eric's ship, he asks where Melody is and draws his sword. When Cloak and Dagger (the manta rays) pull him under the water, he no longer has his sword in his hand.
Factual error: In the birthday party scene Sebastian gets thrown into the air and lands in the birthday cake. A couple of shots later we see that the cake is completely obliterated. There is no way a crab Sebastian's size could create that much damage to such a large cake.
Suggested correction: This is an example of cartoon physics that isn't supposed to reflect reality.
Continuity mistake: When Melody is singing for a moment, as she rises out of the sea on that huge whale, her green ribbon is missing.
Other mistake: When Ariel hugs the adult Flounder, his eyebrows suddenly turn blue, then change back.
