How to break the OS by working with pointers in C?

Question

C++ / C#

How can one crash (any other result) the OS intentionally or unintentionally using pointers?
Dereferencing NULL isn't that interesting, at least not as long as the protection kicks in.

Answer 1

Normally written OS with memory protection - in any way.
A normally written OS with no memory protection -- writing to memory outside the program's address space.
An OS written with bugs -- using those bugs to trick the kernel into writing to memory outside the program's address space.

Answer 2

In XP it was possible, lol, they even wrote code in textbooks.
In modern Windows, no way. If only the curve driver is written (hello Nvidia).