Revealing mistake: When Wally is firing the laser at random from the small spaceship, we see Fidget running down some stone stairs, and there are violent laser impacts directly behind him that could only have hit where they did if they had passed right though his body. (01:41:55)
Revealing mistake: Right after the party splits in the fortress of darkness and Kevin and his piggy friend are chased by the big horned creatures, one of them fires a ball of flame out of its eye. In the next shot when they run trough the pillars of stone, you can see wires on the ceiling, the burning ball is draged with through the set (in the white smoke). This is even better visible some seconds later when the fireball flies trough the door. (01:30:15)
Revealing mistake: When Kevin walks towards the ashes of his parents after the toaster oven explosion, smoke can be seen going back towards the ashes, revealing that it is reversed footage.