Fast and Furious 6
When Dwayne "the Rock" Johnson comes onto the scene in Fast and Furious 6, Roman shouts out "Hey, better hide the baby oil!" Dwayne, who plays Hobbes, immediately comes back with "You better hide your big a** forehead!" This exchange was not in the script, but just some jovial improvising. It was so funny and unexpected that Ludacris spits out his drink onto the table.