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.
Continuity mistake: The first time we see Eric's boat, the number of lamps on the stern decreases between shots.
Continuity mistake: When Eric's statue is about to be uncovered, the sailor behind the dog disappears.
Continuity mistake: When Triton lowers his trident to change Ariel into human, Sebastian disappears, only to reappear later on.
Continuity mistake: After Ariel marries, the rope with which Sebastian knocks out the chef suddenly appears.
Continuity mistake: When Ariel marries, all the guests on the boat suddenly disappear.
Continuity mistake: On the boat, when Ariel goes back to being a mermaid and Ursula cries "Too late!", the whole decoration behind (ribbons, laces, bouquets, book stand, etc.) disappear.
Continuity mistake: Between the moment when the dolphins attack Ursula and when Scuttle does, a fallen table and the mess around appear/disappear/reappear between shots.
Continuity mistake: When Eric and Ursula are about to marry they stand inches away from the Minister. When the birds attack her, the ground-level shot shows nothing two meters ahead of them, therefore the Minister and his book stand have disappeared.
Continuity mistake: At the lagoon, when Eric finds out Ariel's name, the oars and the outriggers disappear.
Continuity mistake: The Minister's cape lays sideways first, and straight behind him in the immediate angle.
Continuity mistake: At the pond, Scuttle jumps on a branch that ends in 2 branches, plus a third one cut. When the angle changes, that third branch is fully grown and filled with leaves.
Continuity mistake: After Scuttle finds out that the girl is Ursula, he flies to tell Ariel. From the aerial angle she, Sebastian and Flounder are gone from the pier, only to reappear a frame later in a closer angle.
Continuity mistake: When Ursula stabs her bobby pin on the mirror it falls backwards. When the angle changes it's straight, then she steps on the make up cabinet and it's back to tilted backwards.
Revealing mistake: In Eric's cabin, when the young girl reveals herself to be Ursula, she starts to dance. Watch her right foot and you'll notice that it's blending and going through a green box as if it were not there.
Continuity mistake: In Eric's cabin, Ursula climbs up the make up cabinet and a purple bottle appears on the right for her to step on. When the angle changes, this bottle, plus a blue one on the left disappear.
Continuity mistake: In Eric's boat, Ursula throws a bobby pin at the mirror, which disappears in further shots.
Continuity mistake: After Sebastian is sunk in the barrel with the laundry, he is drawn up by the lavender, but disappears in the wide angle.
Continuity mistake: During the meal at the palace, the amount of dried wax on the candles increases/decreases between shots.
Answer: Yet another Disney urban legend.
Jazetopher