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.

I love Poirot novels, but one thing that gets predictable and frustrating after a few of them is that there is rarely an actual way for the reader to guess what happened. Usually Poirot does some incredible deduction off the pages, and then presents his findings to both the suspects and the reader.