Poppy's braces disappear during the tree house scene.
When Gru is taking the girls to school for the first time after moving house, the Minions attach the baby capsule to the center rear seat of the van. Then after they arrive at the school the baby seat appears on the rear right seat.
When Poppy is flying away from Maxime's ship, the noise that Principal Übelschlecht's cart makes is that of a propeller aircraft, although the cart is powered by a jet engine.
The vending machine that Ralph gets stuck in contains snack food and soft drinks, but soft drinks are never in snack food vending machines because they aren't refrigerated.
In the 1st movie, Vector got stranded on the moon. And then, he got stranded on Mars in the short movie, Mooned. But in this movie, he was a prisoner at AVL Headquarters.
In the 3rd movie, Silas Ramsbottom retired from the AVL, and was replaced by Valerie Da Vinci. But in this movie, he came out of retirement to stop Maxime Le Mal. Because of this, it's unknown what happened to Valerie.