Revealing mistake: When they are escaping in the beginning of the episode, watch Rodney dial. As he finishes, one of the DHD address panels pops out and falls off.
Revealing mistake: Season 2, episode 2 "Virtu-Date": When Pim is jumping rope with the girl, she is building a card house. When she jumps you can see the tape holding it together.
Duel Monsters Spirit Day - S1-E42
Revealing mistake: At the start of the episode, when each of the dorms are shown celebrating at their respective festivals, all the background extras are shown standing completely still, despite the primary characters moving around.