Ariel's eye color starts out blue and changes to green by the end of the film.
When Melody is rushing to get dressed for the party, her hair is loose, then just before she kicks her bag under the table it's pulled back into a ponytail, then as Ariel enters it goes loose again.
All of the stuff in Ariels treasure cave seem to be intact despite them being destroyed in the last film.
The trident changes in length and thickness many times, and when Melody throws it to King Triton, the cap on the end disappears.
Near the end when Morgana has Ariel entwined in her tentacles, she covers her mouth and it is originally from right to left, then in an immediate close-up it is from left to right.
When Melody was swimming around Atlantica with Tip and Dash, Penguins can only hold their breath for 20 minutes. Walruses can only hold their breathe for about 30 minutes. Being underwater for a long time, Both Tip and Dash would've drown.
At the end, Morgana is trapped in an iceberg that sinks to the bottom of the ocean. Ice floats.
Melody wears her clothes she wears swimming with her ball gown but since they were underwater they should be sinking wet and dripping under the dress she's wearing.
Despite Melody being under water for hours when she comes to the surface her clothes nor hair appears to be wet.
Whenever Melody walks on ice, her feet would've been frozen in a matter of minutes.
Sebastian says he has a voice like the famous opera singer Enrico Caruso. According to the designs of the sailing ships in the Little Mermaid franchise, however, the setting is the 1600s or 1700s, long before Caruso was born.
Some of the character's mouths don't sync well with the dialogue.
When Sebastian lands on the cake it practically splatters everywhere, a crab Sebastian's size should not be big enough to destroy a cake that size.
Several times throughout the movie, Melody is referred to as a teenager. As Melody just turned 12, she would still be in her preteen years.