Other mistake: After the storm, Grimsby helps Eric walk away from the beach while Ariel sings. Every time the shot back goes to those two they are on the same spot, though their legs move as if walking, giving the sense that they're on a treadmill.
Other mistake: In the last scene, where Ariel and Eric are sailing away in the boat, the rainbow in the background that Triton casts is backwards. It goes violet, indigo, blue, green, yellow, orange, and red. Very strange.