It's been a while since I've read them, but Cards on the Table annoyed me the most with it as >!Poirot hires an actor at the end to bait the killer or something!<, but I feel like it happens in most Poirot novels. In Orient Express, for example, there is the part where it's noted that >!a bag wasn't actually covering a lock!<, which is fair enough, but it's hardly enough to deduce who the killer is.