At the ending of Descendants (2015), Maleficent was been transformed into a purple Lizard. In Descendants 2 her color has inexplicably changed to Green. It's likely, however, that since these are her signature colors (green and purple, just like Mal's) this is due to her magical nature.
The first film said that Mal's inability to swim had to do with the island being completely surrounded by the barrier, but there is significant accessible waterfront available to the islanders in this film. Presumably, Mal would have been prevented from accessing this area, but it is an exception to what was previously stated (but then, Mal doesn't control the island and might have limited information as to the extent of the barrier).
When Dizzy is giving Mal her makeover, she is shown putting Mal's hair in curlers. This should have resulted in Mal's hair being curly. But when the makeover is finished Mal's hair is straight.
When Mal and Ben are having a picnic, the shoulder straps on her dress change position between shots. Whenever filmed from the front, the straps are high on top of her shoulder. When the camera angle changes to a far shot from the side, the straps are off the shoulder hanging down her arm.
During the final dance sequence one second Mal's hair is perfectly styled and dry then the next it's soaking wet and flat and then it is perfectly dry and style to again. This happens multiple times throughout the final dance sequence.
Biologically, Dizzy should not be Drizella's daughter, but Anastasia's, as Dizzy has ginger hair like Anastasia, and Drizella has black hair.
When Mal uses her spell book to speed read, Evie comes over and picks it up. When the camera switches between the two, Evie is holding the book differently.
In Chillin' like a Villian, Evie's shoes go from having a high heel to being a flat and back to having a high heel.
This movie takes place six month after the first "Descendants". Yet in both movies it is summer. Enchanted country or not, they should have more than one season.
When Evie is helping Mal with her outfit for cotillion, Mal is talking about all the events coming up and says, "I can't remember what a single one of them is", although you can see Dove Cameron incorrectly mouth the word "are" instead.
When and how does Uma put a spell on Ben? She couldn't have when he was captured on the ship because she didn't have Mal's spell book at that point. Mal doesn't drop her spell book until after they rescue Ben and are on their way back to Auradon. Does Uma have powers of her own? So why would it matter if Uma got Mal's spell book?
After their final practice, Lonnie calls Jay "Jake"
During the song you and me, when Mal is dancing, her hair is straight and wet, but when she is holding hands with Ben, it's dry and perfectly curled.
In the final jousting practice, it appears that Carlos is there but his face is blocked. When you can see the face, it is a different actor.