People put a positive spin on the idea of mob mentality with flash mobs, happy groups that came together in a public place to surprise crowds with choreographed dance moves and songs. Then came cash mobs, when well-intentioned consumers boosted sales at a local store by buying up whatever was in stock. Now the flash mob has taken a sinister turn: A Chicago store owner says his shop is the latest to be robbed by a flash mob, when a group of young adults showed up all at once and stole more than $3,000 worth of merchandise.  According to the Chicago Sun Times, at around 6:40 p.m. on Saturday about 20 people entered the Mildblend Supply Co., a clothing store in a trendy part of Chicago, and started scooping up $200-a-pair jeans -- and then walked out of the store without paying for them.  "I see a group of kids. They're coming in the store in a marching way, like a team," the store's owner, Luke Cho, told the newspaper. "After the 15th or 16th teenager was inside I knew something bad was happening."