Stupidity: After eventually arriving at the stadium, why didn't Ash tell anyone that he was late due to getting kidnapped by Team Rocket? He arrived with half of his Pokémon exhausted and subsequently lost his battle. If he informed everyone about his situation, his battle with Ritchie would likely have been postponed until tomorrow, giving him some much-needed time to rest, which would have given him a bigger chance to win.
Winning Through Intimidation - S1-E15
Stupidity: Yugi summons Gaia the Fierce Knight to the field from his hand. He then uses Polymerization to fuse Gaia the Fierce Knight and Curse of Dragon together to summon the Fusion monster Gaia the Dragon Champion. There was no real reason for him to summon Gaia the Fierce Knight to the field if he was going to use Polymerization on it, as Polymerization allows the player to fuse monsters that are present in both the player's hand and field. (00:09:25)