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?

>>66
If you're on a terminal, the example I gave would work. However, if I changed the underlying device to say a pipe, you wouldn't see any kind of output. Now if I flushed the stdout, I would see the output, regardless of the underlying device.