My vote for “Prank of the Month” (well, last month) goes to the New York-based Improv Everywhere crew for flooding a Best Buy department store with around 80 people dressed almost like their staff.
The full report on this prank shows how the store management couldn’t cope — they didn’t like it, and people get nervous when confronted with something “different”.
It’s also interesting reading the comments on Bruce Schneier’s write-up of this event, where so many commenters fail to see the difference between a “threat to the store” and a “threat to the perceived authority of the store managers”.