The Enquirer says he was knocked flat (dropped like a zapped bat to be exact) by a flying metal sign, that came loose during a violent wind burst while filming in Vancouver. The sign hit him in the back of the head & disoriented Rob & left a knot sized lump on the back of his head. According to the article filming was canceled for the day but Rob returned to the set the next day.
Who knows whether any of this is true or not (put it in the rumor drawer), poor Rob if it is!
Thanks xoxoBaBiiKyKi for the scan.