When Rapunzel and Queen Arianna are riding back to the Snuggly Duckling, there is a brief shot of Rapunzel wearing her original dress instead of her new one.
When Varian and Quirin were fixing a hot water tank, Varian's teal hair streak was gone. But when he and his dad joined the other characters to sing the final part of "Happily Ever After (Reprise)", his hair streak is back. It could be an error made by the animation crew.
Eugene stated that Rapunzel "became the queen she was meant to be." However, her parents are still alive.
When the song Through It All ends, Quirin is not showing to be wearing his gloves.
Also when Eugene tosses a pitchfork to Quirin, the latter's gloves are still missing.
When Rapunzel's hair was cut the insides were still blonde, Which never happened in Tangled.