5. There's A Potato & A Shoe In The Asteroid Field - Star Wars: Episode V - The Empire Strikes Back
Being a production designer on any movie gives you a unique window of opportunity afforded to very few. That's to say, you're one of the people who might attempt sneak things into the movie that only you and your fellow co-workers know about.
And that's precisely what happened during the making of Star Wars: Episode V - The Empire Strikes Back, where the guys working on the asteroid field sequence decided to include a potato and a shoe. Seriously. It's extremely difficult to see properly, but you catch a glimpse of both of these objects soaring past the Millenium Falcon as it tries to avoid a flurry of asteroids. George Lucas himself didn't even know about these hilarious inclusions until years after the movie was released in theatres, and people came out and told him.