This occurs when someone you are talking to on Facebook chat accidentally messages you something intended for someone else.
Me: "Dude I was talking to Stef last night and she said I was good looking for a black guy.."
John: "You're not black..."
Me: "I know... I don't get it.."
John: "A typical example of Facebook Chat Cheating my friend.."
John: "You're not the only one she's talking to ;)"