When you are standing next to somebody either in an Elevator, Checkout Lane, or Bar etc. and they answer their phone via Bluetooth and then you respond.
(In an Elevator a guy walks in)
Stranger: Hey, Whats Up?
You: Not much, just glad the days over....
*Stranger Looks at you like you an idiot*
Stranger: Yea, I will grab some milk on my way home.
You: (Mentally) Crap, he had a fucking Bluetooth and wasn't talking to me. I hope no one noticed. That was severe pwnage by Bluetooth