This time Improv Everywhere sent 15 pairs of twins into a New York subway car and had them mirror each other -- sit opposite each other in the car, sit at the same time, stand, move, eat at the same time, just generally mirror each other's expressions, gestures, movements, everything.
This is closer to actual improv than Improv Everywhere usually does. More often than not, they perform flash mob pranks (e.g., sending a group of men with no shirts into Abercrombie and Fitch, having a bunch of people freeze simultaneously in Grand Central Station, showing up at the gig of an unknown indie rock band and acting like mega-fans and singing along to every song). But this one involves things that more closely resemble theater games -- impulsively acting and reacting to each other in a mirror game. Go here to read a much more detailed description of how they arranged this and carried it off.
As always, the reaction of the bystanders is the most entertaining part.