Magical Express Murder Case: File 2 - S2-E11
Stupidity: Kindaichi's logic as usual proves to be right, however, the theater is surrounded by a pond, not the ocean. The investigation proceeds assuming that the killer who performed a meticulously premeditated murder with great knowledge of technology and conjuring, could never go past a bit of water. Likewise, they say "nobody left their seats since the lights go off", as a certainty, but watching the scene and how it unfolds, it's simply an assumption the plot needs.
Stupidity: The master goes through all the trouble of making Luke the vessel so Luke can go and kill several people and risk getting the attention of the slayer, when all he had to do is have his minions go out and kidnap a bunch of people and bring them to him and he could've fed off them himself.