Few years ago I participated at an international stage. In that time I was 1 kyu and accordingly with our Iwama's school regulation I was wearing an white belt. We were practicing jo suburi and a guy near me wearing a brown belt was doing very bad, pushing the jo without sliding but just gripping it very tight and moving it like pile driver. I told him in a polite tone that he is doing it in a wrong way. He looked at my belt and says " who the fuck do you think you are, a sensei??"
I would have asked him if he was having trouble, if no let him carry on.....