There is having a mirror and using a mirror, I find.
A few weeks ago I passed this guy who was sporting one of these little mirrors. I didn't say "On your left!" (I never do; I just pass as expeditiously and carefully as I can). The fella took me to task and, with a very petulant voice said, "Thank you. Thank you very much for saying 'On your left.'" I ignored him and went on my way. I guess I thought that that is what those little mirrors were for; so that when he saw me coming he would just gracefully move aside to let me pass. Is this an unreasonable expectation?