Berry Scary - S1-E7
Revealing mistake: Blue is walking down a hall with a paddle ball in his hand. He is twirling around the ball and string around the paddle. However it's obvious that the string is somehow just passing through the paddle itself and not going around it. It's especially noticeable when he stops and the string slows down and visibly passes right through the wood of the paddle.
Revealing mistake: After the Shadow Duelist summons Infernal Queen Archfiend, Jaden says, "An Archfiend monster?" and the shot cuts back to the Shadow Duelist saying with that card on the field, every monster with Archfiend in its name gain 1000 attack points. In this shot, take note of the cards he is holding in his hand. He only appears to be holding onto two of the cards, but every other card in his hand appears to be held perfectly fine despite him not holding onto them. (00:12:41)