Inconsistent behaviour in identical code

Question

An error trap trips after running a physics simulation for about 20 minutes. Realising this would be a pain to debug, I duplicated the relevant subroutine in a new project, and

Answer 1

*(long long int *)(&Collision_Axis_Vector.XYZ[0]) = 4594681439063077250;

and all similar lines introduce Undefined Behavior in the program because they violate the Strict Aliasing rule:

you access a double value as long long int