Watch the part where Ariel is watching the sailors on the ship. When Max the dog finds Ariel, he lickes her on the cheek. in the next shot you see Ariel wiping off the other cheek.
When I was 12 and saw Mermaid in the theater, I noticed this slip-up immediately. I turned to my sister, who was 9, and she turned to me, and we both whispered the same thing to each other: "She rubbed the wrong cheek!" So much for excusing them because, "It's a kids' movie!"