Spider-Man 3
The Spider-Man series is famously set in New York City. Unfortunately, fans of the film totally clocked the filmmakers when they realized that one scene, in particular, was obviously not set in New York City. They went and filmed in front of Terminal Tower which is located in Cleveland, Ohio. It was probably way less expensive to shoot in Cleveland than in New York, but still!
North by Northwest
Movie scenes are usually filmed over and over again. One scene in the film North by Northwest consisted of letting off a loud gunshot. Apparently, one of the extras in the background was not a fan of the loud noise and ended up plugging his ears before the big bang. The funniest part is that no one seemed to notice his preparation for the loud noise, and the gesture was left in the final cut of the movie.