Wearing pants inside-out
From Futurepedia, the Back to the Future Wiki
Wearing one's pants inside-out was a fashion statement of kids in the future, as Emmett Brown told Marty McFly in 2015.
Marty had to dress quickly in the clothes that Emmett gave him in order to pass for his son Martin McFly Jr. However, Marty didn't have time to reverse the pants he was wearing, so he simply turned his pockets inside-out and put something in them.
