Hi,
a zero vector normalized does not return a proper normalized vector.
Vector3().normalized returns (0,0,0) ... not of length 1
When you start your code, spin_inertia is a zero vector.
when your code depents on a proper normalized vector better make your own fail safe function
func normalize( vec:Vector3 ):
var newVec = vec.normalized()
if newVec.length() < 1:
newVec = Vector3.UP #or whatever is your default
return newVec
Not tested!