Cops tell us Screech allegedly stabbed a guy after getting into an argument late Christmas evening that quickly escalated-- the bar owner says it was over pics they were taking of Diamond -- and then the actor bailed from the bar. According to court docs ... the victim suffered a 1/2 inch wound under his armpit and bled heavily -- but he didn't notice it had even happened until moments later. Officers located Dustin and his girlfriend in his car not long after ... and the knife was in his ride. Dustin's girlfriend was booked on a disorderly conduct charge.