'Buffy' Star Nicholas Brendon -- Back in Rehab After Bloody Window Smashing Incident

"Buffy the Vampire Slayer" star Nicholas Brendon has checked himself back into rehab after a bizarre incident where cops found him bleeding in an alley.

According to law enforcement sources ... Brendon got into an argument on July 3 with a woman at her apartment, smashed a window with his bare hands ... and then bolted. Firefighters responded and found Brendon in an alley, bleeding from the hands.

Brendon was transported to a hospital, but did not get arrested. He checked himself into rehab 3 days later.