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)
The Gauntlet Is Thrown - S1-E2
Character mistake: When Pegasus shows Yugi the Faceless Mage card, Yugi says, "But my Dark Magician is one of the strongest magic cards in the game." Dark Magician is a monster card, not a magic card. (00:16:50)
Champion vs. Creator, Part II - S1-E27
Character mistake: Pegasus says his toon world cannot be penetrated, but that is not true since it's a magic card, so any card that destroys magic cards can destroy it. Furthermore, monster special effects can also destroy it.
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.
The Master of Magicians: Part 1 - S2-E11
Character mistake: When Yugi activates the spell card Card Destruction, he places the card in his monster card zone and not in the spell and trap card slots that duelists are supposed to do when using a duel disk. (00:17:38)
Playing with a Parasite: Part 2 - S2-E15
Character mistake: Weevil tells Joey that the only monster cards that have enough attack points to defeat Perfectly Ultimate Great Moth are Gate Guardian and Blue Eyes Ultimate Dragon. However, at this point in the series, monster cards such as Dragon Master Knight and Five-Headed Dragon (which have 5000 attack points each) were established to exist and could defeat Perfectly Ultimate Great Moth, which has 3500 attack points. (00:02:29)
Legendary Fisherman: Part 1 - S2-E19
Character mistake: On Mako's second turn, he says, "I'll place a monster face down as well," but then he is shown placing a spell/trap card face down instead of a monster card. (00:13:47)
Character mistake: When Umbra activates Masked Doll, he says, "My Masked Doll will absorb any and all negative effects caused by my magic card." When he says "my magic card," he is referring to Mask of Brutality; however, he didn't activate this card. Lumis was the one who activated Mask of Brutality. (00:16:39)
Character mistake: When Kaiba summons Vorse Raider, he places the monster card face down on his duel disk, instead of face up, which is what he should do if he wants to summon a monster to the field. (00:17:28)
Character mistake: Just before Yugi summons Kuriboh, he says, "I'll place one card face down," but is then shown placing the Kuriboh card on his duel disk. Then he says, "I will summon Kuriboh," but is shown placing a card face down in one of the spell/trap card slots. (00:04:44)
Friends 'Til the End: Part 2 - S2-E27
Character mistake: When Yugi activates the spell card Exchange, Kaiba thinks to himself, "The Exchange card allows Wheeler to take any card he wants from Yugi's deck." This isn't quite true, as the Exchange card allows an opponent to take one card from the duelist's hand, not their deck. (00:09:00)
Awakening of Evil: Part 4 - S2-E40
Character mistake: Kaiba says he will give Joey and Odion five minutes to get back on their feet, and whoever gets up first will be declared the winner. Less than a minute later, the guard looks at his watch and says, "They each have three minutes," even though only 35 seconds or so have passed in real time. (00:08:58 - 00:09:35)
Character mistake: Yugi runs up to a door that includes a four-star monster and is also shown to have been equipped with five face-down cards. After Yugi realises he only has a few cards left, Pharaoh tells him, "Defeating a four-star monster plus four face-down cards is no easy task," but it was established that the door has five face-down cards, not four. Pharaoh even mentioned this a few seconds ago when Yugi first approached the door. (00:12:33)
Brothers in Arms: Part 3 - S3-E19
Character mistake: When Noah uses the Gradius' Option card, he says, "I play the magic card Gradius' Option," but the card appears to be a monster card and not a magic/spell card. He is even showing placing it on one of the monster card slots on his duel disk. (00:09:17)
Clash in the Coliseum: Part 3 - S3-E34
Character mistake: When Slifer the Sky Dragon's attack points go from 2000 to 3000, Kaiba tells Yugi, "But now that your turn is over, Obelisk regains its full strength," but Yugi's turn isn't over; it only just started, as shown by him activating Pot of Greed shortly after. (00:05:08)
Clash in the Coliseum: Part 3 - S3-E34
Character mistake: When Kaiba activates Card Guard, he says he can discard two cards from his hand to protect his monsters from Yugi's attack. However, he only sends one of the cards in his hand to the graveyard instead of two. (00:08:57)
Clash in the Coliseum: Part 3 - S3-E34
Character mistake: When Yugi discards the last card in his hand after activating Spell Textbook, Slifer the Sky Dragon's attack points go from 1000 to zero. This makes Kaiba say, "Now that Slifer's attack points are zero, you'll lose the duel." However, at this point, Yugi has 3000 life points and Kaiba's Obelisk has 4000 attack points, so Kaiba still would have been able to win the duel if Slifer had 1000 attack points, as there would have been a 3000 point difference between the two monsters. (00:18:02)
Character mistake: On Rebecca's first turn, she summons a monster which she refers to as Luster Dragon #2. The monster she actually summons is called Luster Dragon. Luster Dragon #2 is a Level 6 monster with 2400 attack points (which she couldn't have summoned without a sacrifice) and has a turquoise design, whereas Luster Dragon is a Level 4 monster with 1900 attack points and has a blue design (which is the monster that is summoned to the field). She doesn't summon Luster Dragon #2 until later in the duel. (00:11:14)
Flight of Fear: Part 1 - S4-E22
Character mistake: Kaiba uses the special effect of XYZ-Dragon Cannon, which he states that "by discarding one card, he can destroy one of Alister's cards," but he is shown sending three cards from his hand to the graveyard instead of one. Activating this effect does destroy all three of Alister's Tank Tokens, so the correct number of cards is destroyed, despite stating the wrong number of cards. (00:17:20)
Flight of Fear: Part 2 - S4-E23
Character mistake: When Alister activates Toy Robot Box, he says that by discarding three cards from his hand, he can summon three more robots. However, he is only shown discarding one card from his hand. (00:11:40)
