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Impact Wrestling star Jeff Hardy was KO'ed in frightening fashion Friday night in New York ... and TMZ has learned the very painful looking fall left him pretty banged up.

The famous wrestler climbed to the top of a cage during a match at the Manhattan Center ... but when he was knocked off by an opponent -- sources say the fall didn't go as planned.

We're told Jeff was supposed to land on the ground -- instead, he hit the steel steps and was knocked out cold. Hardy eventually regained consciousness ... but was left in a lot of pain.