After Raj makes his "alpha-nerd" comment, it cuts to Leonard, who has his left hand slightly raised and right hand down. It then cuts to Sheldon, and it can just be seen that both Leonard's hands are lowered.
As Sheldon erases the white board toward the end of the episode nothing happens, even though he's vigorously moving the eraser. It would appear this a prop that is not erasable; and is saved, and used in other episodes.