Nelson Thomas was not up to the challenge of a pissed-off husband -- cops say the MTV star got knocked out cold after chatting up a married woman in Las Vegas!!!
According to the police report, obtained by TMZ, Nelson -- who's starred on "The Challenge" for years -- says he met a woman at ARIA Resort & Casino and started talking to her at the valet area, and then ... WHAM!!! Next thing he knew, Nelson was being awakend by someone telling him he'd just been punched in the face and had his chain stolen.