Stupidity: Makes for a funny one-time gag for the benefit of the audience, but it doesn't really make sense that Jay would walk into the hitman's place without knowing he was deaf. It's not exactly the kind of detail that Tommy T. would not mention. In fact, it's the only thing he told the police, apparently, since they, on the other hand, show up knowing he's deaf but do not know he's a big ginger dude and are therefore initially fooled by his squatter story.
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.