What Real Movie Were They Watching In The Movies?

Actually, the real question is "What is most creative way the movies used people watching movies in the movies or used real movie titles on movie theater marquees when something dramatic happpens?" We don't mean fake titles movies, or fake little movies shot especially for the main movie or movie posters or references in the dialogue. Just real movies mentioned on the theater marquees or people watching the film, within the main film in any form. What was the most creative way they used it. Discuss the topic here.

Spoilers are mentioned here!