This is when someone thinks they have hung up the phone after leaving a message, but did not. The person then proceeds to talk badly about the person they have placed the call to, recorded on the message.
Sarah called me, left me a message, and thought she hung up.She did not hang up and I could hear her tell Jenny how much she hated me. I was jerrybrowned.
classic example- and origin- Jerry Brown in his race for California governer left a message for campaign contributions, thought he hung up, but went on to call Meg Whitman(his opponent) a Whore.