A friend just told me about this phenomenon. I think it is stinking hilarious. My friend used to live in NY where this kind of thing happened all the time.
It's called improv everywhere. It's a group of people that improv a practical joke with the intent of making someone's day or giving people something to smile about.
-had over 200 people freeze in place at grand central station for 5 minutes
-given a surprise wedding reception to 2 people that came out of city hall
-gotten a huge group of people to go into Best Buy dressed in a blue polo shirt and khakis to confuse people
-created a mirror with identical twins on the subway
It's very fun.
Here's a video.
The official website is improveverywhere.com.
If you want to laugh or smile, this is a great thing to watch. :)