Unfriending = dirty card

I don't consider myself someone who takes Facebook too seriously, but I'm no stranger to that sinking feeling when I go to someone's Facebook page and see that completely unfriendly "add as a friend" button. How rude! And then you start thinking of the last interaction you had with them.."What could I have done to piss them off this much?" When its ever come up in conversation, it's always the action that's considered too far. "Yea, she told all our friends my secrets, gave me all my stuff back, AND she deleted me on Facebook...can you believe it?"