Have jet engines ever "Sheared" off?
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.