I actually encountered it a couple weeks ago.

Can you spot the infinite loop in this function?

  char* strcpy(char* restrict d, const char* restrict s) {
    stpcpy(d, s);
    return d;
  }

I'll help. A call to `stpcpy` that ignores the return value can be swapped with a call to the (more likely to be optimized) `strcpy`. Since that's infinite recursion, and there is no forward progress, it's undefined behavior and anything goes.

This isn't just theory, it actually broke things in practice for me.