I had my language lawyer hat on. Technically undefined behaviour means literally any behaviour could happen, so in principle you could have a printf that wrote

I had my language lawyer hat on. Technically undefined behaviour means literally any behaviour could happen, so in principle you could have a printf that wrote to random memory locations when you use a bad format string (even without %d) and it would be a conforming implementation. But looking at it now from a more practical point of view, I guess I can't see how that would happen, even with aggressive compiler optimisations (which are the usual thing to trigger really surprising consequences of undefined behaviours).