Then, having completely confused the staff, they just left!
It's all part of a craze that started in the US in June.
Hundreds of people hook up on the net and plan pranks like worshipping a giant dinosaur in a New York toyshop, and then disappear.
No trouble
But the mobsters aren't doing it for anything other than fun.
Trawnee, who is organising upcoming mobs in Australia, doesn't want any trouble.
He said, "If it ever gets out of control, we just won't organise any further events.
"We have no interest in causing trouble - just light-hearted amusement. Spread the word."
Although the flash mobsters are harmless, it can be very dangerous to meet up with anyone you meet on the net.
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