Belgian flashmob performs number from The Sound of Music

Yes, really! I think they must have rehearsed it, separately or together, but it's still amazing! Thanks RH for the link.
It reminds me of an Improv Everywhere stunt.

Matrix ping pong

What can I say? Theatrical special effects are incredible! Thanks Kerry.

Improv Everywhere

Improv Everywhere is a New York-based group which creates drama in public places. One of their most famous missions was sending over 200 "agents" into New York's Grand Central Station, and at the same moment they all froze in place for 5 minutes, while commuters continued to rush past them. In another stunt they gathered a set of pairs of identical twins and placed them in a subway car all facing each other so that their poses perfectly mirrored each other: a Human Mirror.

Fascinating stuff! There's lots more on their website.