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3:20 PM PT -- The brawl video has caught the attention of corporate, a rep tells us the company is investigating the incident. The rep adds, "We do not condone the behavior depicted in this video."

One McDonald's customer learned a tough lesson: DO NOT try to steal from the soda machine, and definitely don't pick a fight with employees, 'cause ya might get your ass mopped.

A video posted Wednesday night shows a young lady getting into a blowout with 2 staffers at an undisclosed McDonald's -- and it's one of the worst ass beatings we've ever seen. One much taller staffer tossed the customer like a rag doll ... literally.

Hair is pulled, faces are smashed, boobs pop out -- and all of that happened to the customer. Spoiler alert: David doesn't beat Goliath this time.