The air conditioner
Once again, I'm amazed about the things we forgive when we love a movie. Major bloopers. The air conditioner clearly visible in the window of a building during the final shoot out/chase. The crane shot that clearly shows modern buildings beyond the backlot. (Did anybody lose their job over these screw-ups?) And there's more. I like Gary Cooper as an actor, but he's way too old for Grace Kelly. The fist fight scene with Lloyd Bridges is pretty weak--as is the entire shoot out with the Miller Gang. And Grace Kelly's acting? Meh.
But we forgive so much. We forgive it all. We love the black and white. We love seeing Kelly and Cooper alive again. Just goes to show: A movie doesn't have to be perfect to become a classic.