NPCs cannot be attacked or cast on
 

a rogue shaman, found wandering around at loc 29, 582, -23 in the Field of Bone, cannot be auto attacked, or cast on. When you try to auto attack, nothing happens. When you try to cast on him, it says "cannot see target". I also tried to cast an aoe spell on a mob next to him, which also did no damage. You can , however, use abilities like kick or bash to hit him.

*edit* The rogue shaman's in the warsliks woods seem to have the same problem