A Duel with Dartz: Part 3 - S4-E35
Revealing mistake: After the Joey knight has blocked Kaiba's attack on the Mai knight, Kaiba says, "Figures Wheeler mess things up," and then inserts a card into one of the slots on his duel disk. The slots on the duel disks have always been shown to be aligned with monster card zones on the front of the duel disk, but when Kaiba sets the card, he inserts it in an area in between where the monster card zones are located, meaning he inserted the card in between the slots, rather than in one of the slots. (00:06:24)