When Cody, C.J., Caroline and Newman stand around the shower talking, a woman in a purple bikini passes behind them twice going in the same direction. She passes by again when C.J. reads to Cody from the mermaid book, and does the same thing.
When Cody and Mellisand converse telepathically, in one shot her hair starts to billow on her left side (facing the camera.) In the next shot, her hair is flatter.
When Todd wakes up in the hospital and Kirstie asks where they'll go, she has her hand on the side of his head with two fingers behind his ear. When the camera angle changes, she has three fingers behind his ear.
When Mellisand swims away after retrieving the locket, the outline of a zipper is visible in the back of her tail.
C.J. worries that if anyone found out about Melissande she'd "end up in a cage like the mermaid in 'Splash.'" It was a tank, since a cage wouldn't hold water, but the thought was there.