Character mistake: Eric says he found the little mermaid sculpture "on the coast of Cartagena" - as the subtitles confirm - but he mispronounces it as Cartageña.
Revealing mistake: Ariel swims underwater and opens a book with stiff and easy-to-flip pages that produce a flutter sound when turned. The paper should be either soggy, or with its pages stuck together or even dissolved, and in no case sound and look as if it were dry.
Other mistake: When Eric holds up the telescope, the cap is on the lens.
Continuity mistake: When Scuttle attacks Vanessa, she scares him away using her left arm. An instant cut later, both of her arms are raised.
Continuity mistake: Vanessa approaches Eric and admires his ring while they stand side by side. A shot later, he has turned 90º and is facing her.
Continuity mistake: During the ring scene, the queen tells Eric to enjoy himself, her left arm lowered. A shot later, it's resting on his arm.
Answer: Her long hair covered her up.
There are a couple of shots after her transformation where you can tell she's no longer wearing her bra. However, I rewatched her transformation scene, and her bra is never seen coming off. It's just gone.