Assume you are trying to traverse through some container. Calling
get_next_element() with a NULL argument will get you the first item
in the list.

Broken:
struct element *x = NULL, *y = NULL;
for (x = get_next_element(y); x; y = x, x = get_next_element(y)) {
    /* Do some processing */
    if (some_condition) {
        /* Note that x is non-NULL here */
        continue;
    }
}

Why it's broken: Even though x is non-NULL at the end of the for loop
get_next_element is called with y still set NULL.

Works:
for (x = get_next_pointer(y); x; x = get_next_pointer(y)) {
    /* Do some processing */
    if (some_condition) {
        /* Note that x is non-NULL here */
        y = x;
        continue;
    }
}