Winning Through Intimidation - S1-E15
Deliberate mistake: In the actual card game, attacking the castle ring would not be allowed, but this is probably done in order to enhance the show for kids.
Yu-Gi-Oh! (1998)
1 deliberate mistake in season 1
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.
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.