It's always sunny in Philadelphia Season 4 Episode 4 : Mac's Banging the Waitress
Charlie finds out that the waitress has been seeing someone. He calls on Mac, his so-called best friend, to find out more information about the guy and to beat him up. However, it turns out that Mac is the guy the waitress has supposedly been seeing. Dennis, heartbroken that he's not Charlie's best friend, tries to win him over.