Have jet engines ever "Sheared" off? I have read that jet engines, at least the under-wing ones, are attached to the airplane by shear nuts and bolts so that in case of unstable forces in or on the engine, the engine would fall off to prevent further damage to the airframe. Have there been any incidents and or accidents where the engine has fallen off the plane in accordance to this design feature?
Engine shearoff, though rare, has happened in a number of cases. 


Boeing 747F experienced a number of engine falloffs:


On Dec 1991, China Airlines flight 358, a 747-200F, lost an engine near Taiwan and crashed.
On October 1992, a El Al 747-200F crashed after takeoff due to engine separation, at Amsterdam, the Netherlands. 
Another 747F from Evergreen Airlines lost an engine over Anchorage, Alaska soon after. 
On October 2004, a Boeing 747-132SF of Kalitta Air lost an engine while climbing and landed without further incident.

Engine separation has been reported on 737s too:


In November 2007, Flight CE723, a Nationwide 737-200 lost an engine during takeoff; the aircraft was landed without further incident.
On December 1987, USAIR FLT 224, B737, lost an engine during climb; the aircraft was landed successfully.



There are cases where engine separation has occurred in B707, B727, and DC-10. aviation-safety.net has a list of engine separation incidents.