Revealing mistake: In the scene after the fire, when Kyle is playing with a candle, as he picks up the candle his belly button is visible. It has been established that Kyle has no belly button, therefore this is a mistake.
Children of Earth: Day One - S3-E1
Revealing mistake: When the children are possessed by the 456, they're supposed to be frozen and stare blankly ahead. But during the second possession, as the camera is panning across a school playground, a boy on the far right is looking directly at it. As it moves closer to him, he swivels his eyes back to where they're supposed to be. (00:16:30)
Revealing mistake: The money that Micah dumps out of his backpack is very obviously fake. In fact, the $20 bills have Benjamin Franklin's portrait on them.
Revealing mistake: When we first see the army of Starks outside Global Dynamics, it's quite obvious that only one of them is actually the actor that plays Stark; the others are merely people who look similar.