Revealing mistake: When we first see Max, his paws are leaning on a balustrade and become transparent. Though it may seem like it's an optical effect because they're in the shadows, the next angle shows them fully lit and opaque.
Revealing mistake: When Max and Eric first approach Ariel and she is behind a rock, there are no footprints in the sand.
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.
Revealing mistake: When Ariel turns into a human for good at the end, Eric kisses her and then they are shown at their wedding. The last shot of the movie is the same shot as this one.
Answer: Yet another Disney urban legend.
Jazetopher