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.
Don't Fear the Reaper - S2-E39
Revealing mistake: After Jaden summons Dandelion in defence mode, he places a card face down. When he goes to insert the card into the slot on his duel disk, his hand doesn't appear to align with the duel disk. His hand appears to pass over the spherical red light on the duel disk, and not the card slot. (00:13:02)