Jeez, calm down. Clipped planchets are uncommon but not rare. I would guess that one might get you $5-10.
It is caused by the coin blank being punched out and the punch running over onto where another blank (planchet) had been punched leaving that "curved clip". If it had been at the edge of the strip of metal, you could have a "straight clip". The Blakely Effect people mentioned is a weakening of the design elements 180 degrees from the clip.