You set a pointer to point to one structure, then you change it and it now points to another structure or array. The compiler doesn't know the semantics of your code, so how can it tell if you meant to do that?

If you changed it via arithmetic or anything other than direct assignment you have violated the standard. Assuming of course that they are part of separate allocations, pointers from one may not interact with pointers from another except through equality testing and assignment.