Yu-Gi-Oh!
Yu-Gi-Oh! mistake picture

The Tomb-Keeper's Secret - S2-E46

Revealing mistake: Whilst discussing Marik with Ishizu, there is a wide shot of the bedroom where Joey says, "What if it's too late to save Marik from the evil that's inside of him?" In this shot, there is an animation error with Serenity's head. Serenity has her head turned to face Ishizu, but the turned head appears to have been poorly pasted over another version of Serenity's head that is facing directly forwards. (00:04:44)

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!

Joey Wheeler, Shadi: Go, Drumsticks.
Mai Valentine: Uh, guys? Joey named his chicken.
Joey Wheeler, Shadi: It's a Nematory, Mai.
Mai Valentine: Whatever.

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.

Seto Kaiba: I'm surrounded by superstitious nitwits.

More quotes from Yu-Gi-Oh!