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?

>>22
"Undefined Behavior" in ANSI/ISO terms is what is known as a "Semantic Error". What that means is that the implementation is free to do anything it wants. In other words, undefined behavior is defined by the implementor and not the standard itself.