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.
Good Luck Charlie (2010)
1 revealing mistake in season 3
Other mistake: When PJ tells Gabe about the attic, he said he had never heard of the attic. But in the earlier episode "Charlie Shakes It Up" Amy tells Gabe to clean out the attic while she's away.