Revealing mistake: In the scene when Amy and Linda sing "Deck the Halls", when Linda steps on Amy's foot, you can easily see that she doesn't actually step on it.
Revealing mistake: When Derek is training Isaac how to fight, Isaac is forced to repeat his approach many times, which results in him being smacked down by Derek. During certain shots, as Isaac is about to plummet to the ground, his hoisting wire can be seen. (00:12:35)
Mewtwo VS Shadow the Hedgehog - S3-E6
Revealing mistake: During Shadow's Super Shadow transformation, half of the background becomes a green screen.
Long Live the Kings, Part II - S3-E19
Revealing mistake: As Boomer fights Kaita, its obvious to see it's a taller stunt double doing the flips and kicks; Kaita is taller than Boomer in all the other shots but they are eye-to-eye during the fight.