Continuity mistake: Arthur is talking to the police, posing as Naomi's fiance: one of the cops asks, ridiculing her lie, "You had your first date at Grand Central Station?" Behind him, a white haired man in a black suit walks down the street carrying a brown suitcase. Next time the policemen are in frame, a different crowd is on the sidewalk, and only after another cut, and more of Arthur's blabbering, suitcase man reappears as if nothing happened. Even better, over half a minute passes and many totally different people...but right at the end of the scene as the policemen wave Arthur goodbye the same man, sans suitcase this time, is there again walking the same way as before. (00:19:45 - 00:20:30)
Plot hole: Hobson passes away in the night, and no one notices until Arthur wakes the next morning, finds her unresponsive and cold, then screams for the nurse. Hobson was hooked up to monitors to warn the nurse (and Arthur) of this very thing, and that nurse would have been making regular checks on her during the night; it's what she's being paid to do. There is absolutely no way it could have played out this way, except in a film that needs sudden drama.
Continuity mistake: In the scene where Hobson and Arthur are in the elevator, and Arthur is looking at the ring; during the second to last shot in the elevator the ring is in the case, but in the next shot Arthur is holding it and putting it on his finger, giving him no time to take it out.