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In one of the dumbest marketing moves of ALL TIME, the Cleveland Cavaliers ran an in-arena video during Wednesday night's game ... showing a male Cavs fan SLAMMING his GF because she's a Bulls fan.

Seriously.

It gets worse ... the man justifies the beating by saying, "Bulls fan? I didn't know you were a Bulls fan!"

At the end of the video, the woman resurfaces with an ice pack on her head ... and wearing a Cleveland shirt.

No word why anyone in the organization would green light this ... but you can only assume someone's gonna get fired.