Inspired by bug 110237 comment 19:

int b, a;

int main()
{
    int *pa = &a, *pb = &b;
    asm("" : "+r"(pa));
    asm("" : "+r"(pb));
    if (pa + 1 == pb) {
        a = 1, b = 2;
        long x;
        __builtin_memcpy(&x, pa, 4);
        __builtin_memcpy(4 + (char *)&x, pa+1, 4);
        return (x - 0x0000000200000001) * 131 >> 32;
    }
}

https://godbolt.org/z/b67zxMv54

On GIMPLE, both store-merging and SLP vectorization are capable of introducing merged long-sized access in place of individual int-sized memcpy's, which is then disambiguated against initial stores on the RTL level, leading to a miscompilation.