The after­math of last Thursday’s pie fight at Powell and Market. Photo by Steve Rhodes.

This year’s annual Valentine’s Day pil­low fight at Justin Herman Plaza. Photo by mary­mad­dux.

The SF Chronicle reports that the city is absorb­ing huge cleanup costs thanks to messes left behind by recent flash mobs. Maybe mob­bers should adopt a “leave no trace” phi­los­o­phy, like that infor­mally fol­lowed by the early mobs, when peo­ple would con­verge, act, and dis­ap­pear, leav­ing only con­fu­sion in their wake. Leaving behind garbage, shav­ing cream, and feath­ers that clog the foun­tains and halt the cable cars is no fun for anyone.