Stupidity: One of the major reveals and plot twists of the movie lies in the fact that a certain character is an accomplice of the medium. The reveal, though, does not explain at all how said individual managed to fool Poirot; the trick briefly shown involves a simple rope pull to open a door. Poirot was looking exactly for that kind of trick, paces around the room constantly on maximum alert, and examines the door itself later. Likewise, it does not explain where and how they could hide the stuffed animal.
Stupidity: Stephanie googles (using the actual Google engine) the name of Emily's fiancé but gets zero results. He is the heir of a mob family embroiled in decades-long feuds, a central figure in the Roman nightlife travelling with private jets, with the newspapers having covered their earlier relationship. Therefore, the chances that absolutely nothing exists online are exactly zero. (00:19:05)
Stupidity: It took Leroy and Charlie too long (16 seconds) to realise the real J. Daniel Atlas is in their apartment, and he's not a hologram. It would take a microsecond for an ordinary human. (We, the viewers, knew immediately.) (00:10:48)