A Duel with Dartz: Part 5 - S4-E37
Continuity mistake: Yugi summons Obnoxious Keltic Guardian to the field and then places one card face down on the table. The face-down card is shown to appear behind Obnoxious Keltic Guardian. Seconds later, when Dartz has Orichalcos Dexia attack, the face-down card and Obnoxious Keltic Guardian are side by side. (00:07:53 - 00:08:27)