Continuity mistake: When the movie starts, the big lamp is lit, yet when Triton arrives to light it with his magic trident, it suddenly appears unlit.
Sacha
Continuity mistake: When Scuttle peeps on the porthole and discovers Ursula, the design of the porthole changes style between the first and last shot.
Sacha Continuity mistake: When Ursula and Eric's boat sails away, people suddenly appear on the left side of the boat.
Sacha Continuity mistake: When Grimsby discovers his crab is gone, little footprints have magically appeared on the table.
Sacha Continuity mistake: When Ariel refuses the eels' help, before they toss the face of the statue, she lowers her head and cries. A frame later the head is tilted to the right.
Sacha Continuity mistake: When the song 'Under the sea' begins, Ariel is sitting on a rock. The amount of purple flowers on the sides, specially on the right side, change drastically between shots: They appear, disappear, reappear, change flowers, then to algae.
Sacha Continuity mistake: When Ariel first sees the fireworks, they are coming from the boat, but when the angle changes and the boat has moved along, they are being fired from the same spot, though now it's set in the middle of the sea, which is absurd. Some animator screwed it badly.
Sacha Continuity mistake: When Sebastian escapes from the dip, the sauce has a white colour. When he throws the bowl over Louis' head it's changed to brown colour.
Sacha Continuity mistake: After the shipwreck, when Ariel sings, moss suddenly appears on the rock right below her hands.
Sacha Continuity mistake: Eric's ship sails swap from plain to striped when the storm breaks.
Sacha 
Continuity mistake: When we first meet Max, his front paws swap from white in the front angle, to grey in the angle shot from behind.
Sacha Continuity mistake: During the Sebastian and Louis scene in the kitchen, all the stuff on the tables around keeps changing (appearing, disappearing, reappearing) between shots, with no logic whatsoever.
Sacha 
Continuity mistake: The lettuce under which Sebastian later hides, wasn't on the table in the first place.
Sacha 
Continuity mistake: The bowl in which the chef dips Sebastian swaps from grey coloured to blue.
Sacha Continuity mistake: When Flounder guides Ariel to the sunken statue of Eric, the statue's visage and position change drastically at least 3 times during the scene.
Sacha Continuity mistake: The first time we see Eric's boat, the number of lamps on the stern decreases between shots.
Sacha Continuity mistake: When Eric's statue is about to be uncovered, the sailor behind the dog disappears.
Sacha Continuity mistake: When Triton lowers his trident to change Ariel into human, Sebastian disappears, only to reappear later on.
Sacha Continuity mistake: After Ariel marries, the rope with which Sebastian knocks out the chef suddenly appears.
Sacha Continuity mistake: When Ariel marries, all the guests on the boat suddenly disappear.
Sacha 
Chosen answer: They are all throughout the film but the biggest one is when Eric is steering the ship to stab or skewer Ursula.
Disney-Freak