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?

However, to be fair, determining them by fully understanding the compiler is a bit of a pain, and in some cases it would be literally impossible(let's say you decide to defeat my argument by using a "true random" quantum RNG), the typical way one determines what the behavior is, is by examining the assembly listing generated by the compiler or just disassembling the compiled program - it would be a lot less work than mentally following the same steps a compiler would have to.
How do you know the compiler produces the same program every time? Remember that the implementation is free to do whatever it wants with undefined behavior.