This video really made my day. Improv Everywhere, which stages synchronized outbursts of surrealist performance art, recently organized a musical in the food court of a Los Angeles shopping mall:
We used wireless microphones to amplify the vocal performances and mix them together with the music through the mall's PA system. We filmed the mission with hidden cameras, mostly behind two-way mirrors. Apart from our performers, no one in the food court was aware of what was happening.
Link: Food Court Musical