The Hong Kong Kowloon Treasure Murder Case File 3 - S1-E3
Stupidity: The plan of the murderer involved tying a wire between two different building. To do that, they needed access to the victim's apartment, and to set the trick in advance. The problem is that they tied the wire to a pillar in the middle of the apartment, below eye-level and well in advance; since they spent basically the whole afternoon with Kindaichi (and they met him by chance) the timing of the plan wouldn't work. He'd have literally walked into the trap just coming back home.
