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: When the duel begins, the Shadow Duelist goes first. He has five cards in his left hand and draws a sixth card from his deck. He then places the sixth card next to the fifth card, and then picks up the fifth card in his hand, but the sixth card remains in the same position, without its position being affected by the fifth card being picked up. (00:12:32)