Fridge Logic: If you're going to lure somebody to the middle of nowhere to kill him, why not do a drive-by instead of having a minion attack him from a crop duster? Hitchcock himself acknowledged the scene makes no sense, but justified it since nobody notices while watching the film.
Hitchcock himself developed a case of it for the scene where Eve and Leonard covertly talk using two phones at an airport. He held off filming the scene for a while, then apparently came up with a way it was possible and shot it, though he never told anyone what it was.
Idiot Plot: The plot of the movie wouldn't have happened if the bad guys had just shot Roger the first time they wanted to kill him instead of hatching a complex scheme to get him in a car accident. Later, they get another opportunity to assassinate him, but they again ruin their own plan by using a loud, highly visible crop duster that gives him ample warning time instead of just doing something simpler like planting a sniper.
Vandamm crosses it early on when he refuses to listen to Thornhill insisting that he is not the man Vandamm thinks he is and decides to simply kill him despite his protests, as if implying that whether Thornhill is telling the truth or not, it makes no difference to him.
Leonard crosses it in the climax when he steps on Thornhill's hand, the one he's holding precariously on to the cliff by (Eve is holding on to his other hand), after Thornhill asks him for help. A Karmic Death follows moments later.
"Seinfeld" Is Unfunny: A modern viewer who has never seen the film before can recognize trope after action movie trope, perhaps unaware that it was a defining film in that genre and that films like James Bond and Indiana Jones were heavily influenced by it.
Spiritual Adaptation: James Bond was still a popular book series when this film came out, but it essentially codified many of the elements that typify Bond. Morally ambiguous Femme Fatale spies, Affably Evil supervillains, sexually ambiguous dragons, use of Monumental Battle and wild action sequences. Likewise, Sean Connery's turn as Bond, as a more suave individual than his literary counterpart was based on Cary Grant's performance in this film. Indeed, Hitchcock who initially considered adapting James Bond was upset that Albert Broccoli (who he knew personally) ripped off his film for many of the early Bond films.
What an Idiot!: Roger Thornhill goes to the UN in New York to speak with Lester Townshend and find out who was impersonating Townshend. One of the Big Bad's mooks throws a knife at Townshend, causing Townshend to collapse into Thornhill's arms. You'd Expect: Thornhill to leave the knife as-is, and yell out, "Get a doctor! He's been stabbed! Someone threw a knife at him!" Instead: Thornhill yanks the knife out of Townshend's back, getting his prints all over the knife and looking very much like he'd done the deed. A photographer catches his picture while he's holding the knife. Even Worse: Thornhill bolts from the building and goes on the lam, making his innocence look even more in doubt. However: The whole scene played out so quickly that Thornhill's reactions seemed to be purely instinctive rather than based on any kind of reasoning.