I remember. It was quite a discussion on Rocketry Planet.
(I am a X15 freak)
The X-15 recall started before that accident.
I think the X-15 was recalled because the way the motor was retained left a gap in the back which let out enough of the ejection charge that deployment of the 'chute wasn't reliable.
I still have one, but I glued the retaining ring into the motor mount so that no one can attempt to fly it,
Wow. The recently retired Senior Pastor of Arapaho UMC is a friend; I'm going to have to ask him about this one next time I see him. I wouldn't have thought that anything propelled by an 18mm motor would have that kind of penetrating power.