At the end of the second movie Dizzy gets an invite to go to Auradon but in this one she's still on the isle waiting to be chosen.
When Audrey gets defeated by Mal towards the end, there are several mistakes here, the first is Audrey's body position when she's lying on the ground, second when Audrey falls to the ground she's still wearing the crown and holding the scepter in her hand, in the next shot both the crown and scepter are nowhere on site, finally without any explanation Audrey has suddenly changed back in her regular appearance before turning evil.
When Mal makes her confession, when Evie, Jay, Carlos, Dude, and Ben get turned into stone, Ben has a beard and mustache. When the spell is broken after Mal defeats evil Audrey, Ben is clean shaven like he is for most of the movie.
When Audrey sends the people at the birthday party to sleep, Jane swims in the lake because the water undoes any spells. However, in the first movie, Ben swims in the lake while Mal sings If Only and he is still under the love spell.
During the last song "Break This Down" Evie's shoes change back and forth from ankle boots to the sneaker wedges.
When the VK's are leaving the Isle after collecting Hades' ember, the body double for Celia is visible, rather than the actress Jadah Marie.
When Mal gets the Fairy Godmother's wand to take down to barrier between Auradon and the Isle, she is wearing a floor length purple dress with a slit. The camera then goes to a shot of Uma walking onto the bridge. When the camera cuts back to Mal hugging the Fairy Godmother, she is suddenly wearing a short midi-dress version of the same purple gown without any explanation as to why her dress changed.
Uma's boots constantly changes in between shots during the song "NightFalls".