I have not participated in a flash mob, but I do paricipate in unexpected acts of kindness.  My favorite is to purchase Starbucks for the car behind me in the drive thru oe the person behind me in line!  I know the baristas, so I know that they follow through, plus I use my Starbucks card so I get the receipt which then eliminates the order from their system.  I once drove up and the Barista said, "This one is on me!"  I told him thank you but that wasn't necessary.  He then infomed me that he had been behind me in the drive thru a couple of weeks earlier and I had paid for his drink!