Revealing mistake: When Eric and Ursula sail away and leave Ariel crying, the boat is seen far away with no ripples drawn underneath, not even a shadow, giving the sense that it's floating or, literally, flying.
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: 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.