There has never been a crime committed using a registered machine gun.
In 1988, an Ohio police officer used a MAC-11 from his department to kill and informant. There was at least one other involving a doctor, if I recall.
In Targeting Guns, Gary Kleck interviewed an ATF official, who claimed there had been less than 10 crimes committed with machine guns since the inception of the registry. However, even those were most likely procedural violations (ie. bad paperwork).