I saw a video, but I don't remember when (last year?) of a small group of people in a public place (Paris?) who had a boombox and all started dancing, and then more and more random people joined in the dance until they had this big mob of dancers and then they all broke off at the same time.
And now I can't find it! Because 'dancing in Paris' doesn't really help in Google and I'm not sure how to search for it.