"Flash mobs" have become widely known as the pre-planned meetings of large groups of people to spontaneously dance.

A note is sent out of where the event will take place, and followers show up to dance on cue.

But a recent trend, called flash "robs," has dozens of people converging not to two-step to a Michael Jackson number, but to rob stores.

The planned thefts are organized through some type of social media. And like they're dancing counterparts, flash rob videos are popping up on YouTube from across the country.

"As you saw on their faces, they thought it was pretty funny. But we sure don't," said Suzanne Ziegler, an employee of City Shop -- a store that was flash robbed.

Police are also concerned. Authorities warn this type of incident can easily turn violent when the "mob mentality" takes over.