There’s nothing hotter than a hot microphone. Despite decades of cautionary tales, politicians persist in falling into the trap of having their not-made-for-public-consumption words broadcast to the world.

The latest installation in the anthology of live-microphone moments came Monday, when President Obama told Russian President Dmitry Medvedev he’d have “more flexibility” on nuclear missiles after his presidential election. But this was hardly a first.

●President Ronald Reagan may be known as the Great Communicator, but he wasn’t immune to a microphone gaffe. During a sound check before a 1984 radio interview, he warmed up: “My fellow Americans . . . I’ve signed legislation that will outlaw Russia forever,” he said. “We begin bombing in five minutes.”...