19. The "Smoking Required" Sign Outside Biff's Museum
Later in the film when Marty returns to 1985, he discovers that Biff (Thomas F. Wilson) has used the sports almanac to make himself one of the richest and most powerful men in the United States.
Hill Valley is a morally bankrupt hellhole adorned with monuments to Biff, including the Biff Tannen Museum, which in addition to featuring a creepy wax-work of him in the entrance window, has an especially tacky sign for those who wish to come inside.
The sign says "Smoking required," and while it might seem like a mere joke on the face of things, it's easy to picture Biff of all people insisting that people can genuinely only enter the museum with a lit cigarette.