The Alchemy Murder Case File 4 - S1-E9
Stupidity: Everyone mentions in the first episode that the various doors are made out of different metals (some rather unlikely; a door made out of pure manganese?) but they don't show it, all have the same color and weight (the door made out of lead and the one made of tin would be significantly different, and one is the treasure, 966 kg). More importantly, since it's a known fact that the doors are made out of different metals, each has a number that matches the periodic table, and they are looking for gold, the supposedly hard enigma concocted by the genius alchemist is something a high schooler would figure out in minutes; Miyuki even knows its atomic number by memory when randomly asked.
