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?

>>40,42
What >>43 said. Compiler could refuse to compile programs with undefined behavior. Maybe it's not the best thing to do, but it's about as reasonable as the example someone gave where it's generating random code.