The Heart of the Cards - S1-E1
Character mistake: When the duel begins, Kaiba says, "I attack with the Hitotsu-Me Giant," but once the monster is summoned, he doesn't attack Yugi with it. (00:10:20)
Yu-Gi-Oh! (1998)
1 character mistake in The Heart of the Cards - chronological order
Starring: Eric Stuart, Amy Birnbaum, Dan Green, Darren Dunstan
Genres: Action, Adventure, Animated, Family, Fantasy, Thriller
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.
Joey Wheeler, Shadi: Go, Drumsticks.
Mai Valentine: Uh, guys? Joey named his chicken.
Joey Wheeler, Shadi: It's a Nematory, Mai.
Mai Valentine: Whatever.