Writing outside an array will corrupt whatever object is stored there, if
any. It could be part of the heap's internal fields, it could be another
object, it could be unallocated memory, or it could be inaccessible memory,
in which case, you'll get an access violation. So don't ever write outside
an array's storage.

That said, if you have a pointer p to an element inside an array, and that
element is at index 2 or beyond, then p[-2] is a valid way to refer to the
item 2 positions before *p.