Okay, in The Bridge on the River Quai, at the end, when William Holden gets killed, Jack Hawkins gets defensive with his Thai women bearers, and protests that he had no choice. What? To kill Holden with his mortars? No, Holden was killed by Japanese rifle fire. I never got that scene.
In NBNW, the plot from that point on depended on Cary sitting across from Eva in the dining car. So how could that be "arranged" on purpose? And in the very beginning, the fortuitous event of Cary just happening to be there when the victim is knifed and then getting the knife in his hand... But I didn't care! It was exciting, I like Cary Grant, and it was a good movie!
In Heat, with Pacino and DeNiro, when DeNiro's crew is coming out of the bank they just robbed, Pacino, who's heading to the bank but is still blocks away, somehow can see that far and says, "They're already coming out of the bank!" And then at the end, from a similar distance away, he sees DeNiro's girlfriend waiting for him in the car. And then, when DeNiro is running away, Pacino just happens to guess right on the direction he took. But I still liked that flick! The shootout scene in the streets of LA - based on a real-life shootout, I've heard - is awesome. Great acting by all concerned, including Val Kilmer.
I'll think of some more, given the time. The point is that, though some WTF moments occur in films, the good ones don't let those moments ruin the movie.