The MediaCoach Law of MIcrophones states "Any microphone, at any time, is live, and will pick up an unguarded remark".
Alas, Gordon Brown forgot The Law today in Rochdale. After a five-minute discussion with a local resident, Gillian Duffy, he turned to get into his car, saying "It was nice to meet you".
However, once he thought he was out of earshot of all but his closest aides, he was recorded as saying "That was a disaster. You should never have put me with that woman. Whose idea was that?"
An aide responded "What did she say?"
Mr Brown replied: "Oh, everything, she's just a sort of bigoted woman who said she used to vote Labour."
The conversation was recorded and reported by Sky News. It's probably the biggest gaffe of the campaign so far. The Prime Minister has since apologised, when interviewed by Jeremy Vine on BBC Radio 2, after listening to a recording of his remarks.
The Tories have siezed on the issue to make political capital. Nick Clegg, the LibDem leader has declined to comment - wisely in my view.
The incident is an embarrassment. However, it's hardly surprising, given the stress of the campaign. I know that in their suppsed "private" moments, members of all parties express frustration with people. I've heard many of them say things (as we all do at times) that we wouldn't want recorded and broadcast. Unfortunately for Labour, it adds to the perception of an out-of-touch Government past its sell-by date.
John Major had a moment in a TV studio when he referred to three cabinet colleagues as "b**tards". During his 2000 presidential campaign, George W. Bush called New York Times reporter Adam Clymer a "major league a**hole" just before a speech, but not far enough away from a mike.
So remember, the mike is always on. The camera is always rolling, and there's always someone in earshot