Yu-Gi-Oh!

Tomb of the Nameless Pharoah - S5-E15

Stupidity: When Solomon and the guide reach the room containing the Pharaoh's treasure, Solomon begins walking across the bridge to reach it. Once the guide realises that the bridge isn't booby trapped after Solomon walks partway across it, he attempts to kill Solomon and take the treasure for himself, but he doesn't wait until Solomon has fully crossed the bridge, leading to him setting off a booby trap and getting himself killed. (00:11:27)

Casual Person

Duel Identity, Part II - S1-E30

Character mistake: Yugi mistakenly says if he doesn't get a monster out to defend himself Mai can attack him directly, which wouldn't be possible since the rules of Duel Monsters during the Duelist Kingdom arc forbade direct attacks on defenseless opponents. That rule wouldn't be implemented until the Battle City arc.

More mistakes in Yu-Gi-Oh!

Mai Valentine: Hey! Cross on the green, not in between.
Duke Devlin, Raphael, Valon: Sorry, but we're being chased by a bunch of robed weirdos.

More quotes from Yu-Gi-Oh!