I'm not sure if they have before in the Paris finishing circuit, but it happens a lot in road races.
I guess the peloton would still carry on and do the extra lap, because their time would be taken and they could make up quite a bit while Ineos are faffing round with their champagne.
I'm pretty sure this is a dumb question, but hey ho:
Breaks often get away a bit in the final laps of the Paris stage; has anyone ever actually gained a lap on the peloton? What happens if they do?