A situation where a conversation not meant to be heard by a third party is recorded on voicemail or answering machine, due to one participant dialling a number (perhaps unintentionally) and forgetting to hang up after the call was not answered. The voicemail then activates and records the conversation. The person, on whose voicemail the conversation was recorded, can then listen to the conversation he/she was not meant to hear, and if necessary, provide it as evidence.
Such voice messages are often of poor sound quality, due to cell phone being placed in a pocket or purse, but often can be clear enough for the listener to extract the main idea.
Kate: So, I've heard you're going out with Adam? That's just two days after I broke up with him!?
Lana: Ahm... ... Who told you?
Kate: I heard your whole conversation with Carol on my voicemail. You called me, and must have forgotten to hang up after I didn't answer.
Lana: F***! I can't believe it got caught on voicemail!