In a television show or radio program, a controversial or exciting segment designed to get people talking about the show.
We spent three or four hours very late one night talking about how a programme can basically market itself.' This involved establishing one or two 'watercooler moments' which would get people talking the next day.
—Simon Garfield, "A storm in the airwaves," The Observer, July 1, 2001
1991 was the Year of Pee-Wee, the year the disgraced children's TV show host was busted for a solo act of indecent exposure in an adult movie theater. He made his triumphant return on the MTV stage, clad in his trademark tight gray suit. And from then on, the [Video Music Awards] were on their way toward becoming destination viewing, a yearly event that promised the hot bands and babes and boys of the moment, plus at least one watercooler moment for young fans to hash over the next morning.
—Jennifer Weiner, "MTV's awards show produces excess," The Philadelphia Inquirer, September 8, 1999
It's a classic workplace scene: Two or three co-workers arrive at the office watercooler more or less simultaneously and, people being the social creatures that they are, a brief and ideally non-work-related conversation ensues. Nowadays, these confabs are just as likely to break out in the coffee room, alongside the photocopier, or while waiting to use the fax machine. But the "watercooler" was long ago chosen as the symbolic location for spontaneous workplace chinwagging.
That's the idea underlying today's phrase. Insert some gratuitously controversial or titillating content into a show, and the next day the watercooler conversations will begin with the phrase "Did you see/hear last night's episode of X?"