I was in my dorm's laundry room and a guy needed to get past me and said "Excuse me, sir". I knew if I spoke it would ruin the moment for me, because normally when I speak after someone calling me 'sir' I get the "sorry I thought you were a guy" or something of the sort. So I didn't speak. Simple as that.
And to begin with I don't get called 'sir' very often, because I'm pre-T and only bind and dress to look masculine. And even binding doesn't seem to help all that much.