Theres been alot of these happenings
The group is called improvise everywhere made up of pple from all over the world.
They gather to excute their mission and leave almost immediately after its been excuted.

http://improveverywhere.com/

This was the interesting one that caught my sight 3 years back.
http://improveverywhere.com/2005/03/19/look-up-more/


singapore's Version

Imo. Failed quite badly. But still good effort trying