There are very few duelists in the world of Yu-Gi-Oh! who are as strong as Yugi Muto. The undisputed King of Games is incredible at what he does. He isn't perfect, though. He's made plenty of mistakes during his duels throughout the series, both in his decisions and deck building.
He's made small mistakes and big ones. Whether they were trusting a stranger a bit too much with his cards or literally attempting to steal someone's soul, they certainly impacted Yugi's dueling and his life.
Using The Seal Of Orichalcos
One of Yugi's biggest mistakes of all time is well known, it's using The Seal Of Orichalcos during the Waking the Dragons arc of the series. He used this card because he thought that he was backed into a corner.
When he did this, he gave in to the rage of his heart and was even willingly sacrificing the spirits of his monsters knowing that they were real. In the end, Yugi paid the price for Atem's mistake by giving up his soul.
Ignoring Yugi
In both the case of using the Seal of Orichalcos and a point that will be delved into a bit later on the list, Atem did not listen to what Yugi wanted.
He acted on his own wishes without consulting his partner. Both times that he did this, the consequences were deadly. While the two managed to overcome both of these issues and move on, Atem should have listened to Yugi no matter what he thought was best.
Letting Weevil Hold Exodia
While this didn't exactly happen during a duel, Yugi should have never trusted Weevil Underwood, no matter what. Because he trusted him with holding his rarest cards, he lost Exodia forever.
Weevil Underwood is trash, and luckily, Yugi learned this fact right after he threw Exodia into the ocean. However, it would be a very different story for the duelist if he had been able to keep Exodia in his deck and use it against some of his strongest opponents of all time. Perhaps he would have gotten through some tough moments a bit easier.
Using Beaver Warrior
The Duelist Kingdom era of Yu-Gi-Oh! was a weird one. Rules were very fluid and in flux, there were no tributes in order to summon strong monsters, and duelists could basically just make random claims that would become fact.
And despite strong monsters being easy to summon, Yugi used weak monsters like Feral Imp and Beaver Warrior in his deck. What was the point of these monsters? He should have filled his deck with stronger ones, instead.
Constantly Cheating
Yugi cheated quite a bit. Whether it was using cards like Multiply completely incorrectly or living through attacks that should have wiped out his life points, Yugi refused to play fair.
Sure, this served him very well, but it really invalidates some of his wins. He should have lost all of his life points during the Pyramid of Light movie, for example, but he won that duel in the end.
Big Shield Gardna
Yugi used a card called Big Shield Gardna in most of his decks throughout the series. It's a four-star monster with 2600 defense points, which is a great statline. However, once it's attacked, it will switch to attack mode. It only has 100 attack points.
In real life, this weakness would be exploited quite often. However, in the show, no one ever attacks Big Shield Gardna, and it acts as an easy stall tactic any time Yugi needs a few turns to think of a plan. Lucky for him, at the least.
Using Black Luster Soldier & Magician Of Black Chaos
Yugi uses Magician of Black Chaos and Black Luster Soldier in his deck. Both of these ritual monsters have a stronger effect monster forms, Dark Magician of Chaos and Black Luster Soldier - Envoy of the Beginning, respectively.
Both of these monsters actually had effects and were easier to bring out. Yugi really should have used these monsters instead of the versions that he used, but he went for the underpowered versions instead.
Literally Murdering Weevil Underwood
Sure, Weevil sort of brought this upon himself by taunting Atem and pretending to have ripped Yugi's soul apart, but the Pharaoh went ballistic on Weevil, attacking him over and over well after the duel had ended during their match in the Waking the Dragons arc.
It was actually bringing Atem some joy. While Tea manages to get him calmed down and stop attacking, he showed no signs of actually doing it on his own. Deserved, but still not a great look.
Willingness To Let Kaiba Die
During their match in Duelist Kingdom, Atem was willing to let Kaiba die if it meant that he would win the duel.
Sure, he was doing this because he needed to save Yugi's grandpa, but this never should have been the answer. Yugi managed to get Atem stopped before any real damage was done here.
Dark Magician Cards
Despite using Dark Magician so frequently throughout his decks, Yugi didn't really pack Dark Magician cards all that often.
He packed cards like Thousand Knives and Dark Magic Curtain rarely, and left other cards (which there are plenty of others) out of his deck completely, which isn't the smartest move.
