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Rob Gronkowski scared the hell out of New England Patriots fans last night ... when the superstar tight end BODY SLAMMED his friend on stage at a Las Vegas nightclub ... landing hard on his broken forearm ... and TMZ has the footage.

Gronkowski -- who signed a $54 million six-year contract extension in June -- broke his forearm last season, then injured it again during the Pats' first playoff game against the Texans.

But that didn't stop him from executing a Stone Cold Stunner-esque move on a friend during a playful dance battle at XS nightclub last night.

As Gronkowski was throwing his friend to the ground, the tight end -- who was wearing a protective sleeve on his busted wing -- also crashed to the ground and smacked his damaged arm on the stage in the process.

Gronk popped right up and waved his arms around -- a sign that he didn't reinjure anything during the stunt -- but seriously, HOW DUMB WAS THAT? Gronk popped right up and waved his arms around -- a sign that he didn't reinjure anything during the stunt -- but seriously, HOW DUMB WAS THAT?