Program won't crash when it should

(GCC C)
I ran a pointer over each member of an integer array, storing one character of input in each cell.

The program seems to consistently crash when I try to store X+3 or more characters, where X is the size of the array, but if I do not exceed X+2 it will not crash.

Why does it crashes exactly at the point it does, but not for X+1 or X+2?

>>18
You have the C standard and within that, the behavior his program will have is undefined, however while that wouldn't be conforming C, it doesn't mean it doesn't have very precise semantics when using a specific compiler with specific settings on a specific architecture - you can almost always determine them when you have fully specified the environment, and when you can determine them, undefined behavior will be fully defined.
What if the compiler has non-deterministic behavior? What if the compiler has a setting like "optimize for 30 seconds" which depends not only on the compiler but also the speed of the machine, the total amount of load on the system, disk swapping and other non-controllable factors?