Debugging seg fault caused by buffer over run

I have the following code:

if (destIndex < destLen && srcIndex < srcLen)
    dest[destIndex++] = src[srcIndex++];

This code runs fine.

If I remove the if, then the code crashes.

If I then run the following code

if (destIndex < destLen && srcIndex < srcLen)
    dest[destIndex++] = src[srcIndex++];
else
   fprintf(stderr, "Buffer Overrun\n");

Then I never get the print out "Buffer Overrun"

SO, how to debug this?

The seg fault only happens with optimizations turned on (no debug mode)