Shito! Kyofu no kokushiken - S1-E11
Stupidity: The 'repairing the cloth' scene does not make any sense as presented. Shiryu cuts his wrists and just pours blood on the armors spilling it before he reaches them (one just assumes that Mu told him off-screen that he needed to bleed all over the armors and not collect it, would have been pretty awkward if he died hemorrhaging before Mu could fetch a bowl), and he keeps saying that he just needs to repair Seiya's armor. Since Mu says that he needs half of Shiryu's blood to repair BOTH armors and he'll die if he loses a third, Shiryu could repair Seiya's armor and go fight armorless - which he always does anyway, he's arguably stronger without it - giving just 1/4 of his blood, instead of dying leaving Seiya on his own. No need to expect a character from a 'shonen' to act logically, but simply a throwaway line saying that he needed to repair his own armor out of duty to his stars or something similar, would have been enough.
Ayaushi shiryu! Cloth no hakaba - S1-E10
Stupidity: Saori blushes when Seiya opens the door shirtless...but she's seen him shirtless practically all her life, the boys kept training and fighting all the time. He even fought shirtless for the last part of his duel with Shyriu just the day before.
Yomigaere! Eiyuu Densetsu - S1-E1
Stupidity: Despite being in training for 6 years at the temple to compete for a magical armor consecrated to a constellation and meant to harness the power of the Cosmos, Cassios and everyone else but Marin (Shaina towards the end) are mistified about the whole idea when Seiya uses it. What in the world have they all been training for, for 6 years, then? Shaina has to be the worst trainer ever. Cassios even calls it a 'superstition', but his master Shaina channels the power perfectly. It's like a Muggle working at Hogwarts and not believing in magic.
Doragon! Muteki no ken to tate - S1-E4
Stupidity: The tournament bracket of the Galaxian Wars makes hardly any sense, in particular in the anime where there is no explanation for its creation and why some fighters have a "bye"; the preliminary round is not over yet, two fighters have no opponents and one is missing, but they rush a fighter who battled just 2 days before, against someone fresh.