It was one of the most shocking moments in the entire catalogue of Friends episodes - and it led to the series being completely rewritten.
Monica and Chandler's tryst in a hotel room at Ross's wedding was only ever meant to be a one-off, but fan reaction led to writers giving them a future.
"We had no idea what response that was going get," said co-creator Marta Kaufman said at the ATX Television Festival.
When the moment unfolded in front of a live studio audience, they were so shocked to see Monica pop up from underneath the covers with Chandler that they started screaming, and filming had to be stopped.
"We had to stop the show because people were screaming," said Marta.
"We thought it was going to be funny and we were going to get rid of it.
"Suddenly the audience told us they had been waiting for that and we had to rethink how we were going to keep going and change the relationship.
"But that wasn't a person — it was an entire, immediate expression of joy."
Fans told the creators that they had been waiting for that moment for a long time and thus the relationship took over from Ross and Rachel as the central relationship in the show.
Marta had previously revealed that Monica and Joey had initially been intended as the main couple on Friends because "they just seemed the most sexual of the characters".