Improv Everywhere brings participatory art (read: prank in public places) to a new level. In this one "mission", 80 people showed up wearing blue polos and khaki pants and "infiltrated" a Best Buy store.
With our main photographer busted, I took out my camera and started taking covert snapshots. One employee caught me in the act and rushed over. As soon as he got to me, I caught him off guard with a question, "Hey, do you know where I can find the right memory cards for my camera?" He stammered for a second and then said, "Sure. They're right over there." I thanked him and was on my way. Another employee caught me moments later in the DVD section, but I disarmed him with a question as well, "Do you know how much the Star Trek DS9 DVDs are? There is no price tag." We chatted for a second about how expensive the set was, and by the time I walked away he forgot all about the camera.
Link - Thanks Anna!