The Alchemy Murder Case File 3 - S1-E8
Revealing mistake: The setup of the story is the usual; characters forced to coop together at a secluded location, they can't contact the outside world and one of them is a killer. In this case, they are on an island and the only radio they had been on the boat that someone set on fire. However, when one of the suspects shows Kindaichi a photo from their smartphone, the screen shows that they have an active 3G connection. (00:08:00)
Revealing mistake: The bullets the priest takes out of the ammunition tray have no primers. (01:01:33)
The Delivery - S7-E6
Revealing mistake: Eddie's bar has just burned down, and everything is ruined. However, the glasses above the bar are pristine and clean. At the very least, they'd be blackened and smeared by the smoke and soot, even if the heat of the fire hadn't destroyed them. (00:10:28)
Revealing mistake: As the RCV enters the gravel field with the bomb, you can see a person in shorts walking casually in the background, despite the area being sealed off.
Revealing mistake: In the scene where the wife comes in to the morgue to view her dead husband, who she helped to euthanize, the dead man is breathing throughout the whole scene. You can see the sheet rising and falling at his stomach. (00:22:15 - 00:24:05)