Bad person enters a build and walks to the reception.
BadPerson: Hello there, My name is badMAC I would like access to the rooms beyond the reception!
Reception: Sorry, no. That is off-limits for you.
Bad person sits down and listens.
Someone else enters the same room and walks to the reception.
Someone: Hello there, My name is ValidMAC I would like access to the rooms beyond the reception!
Reception: Sure. Go ahead mister ValidMAC
Bad person remember the name, waits until the other person is out of sight and walks a second time to the reception