I find being assertive takes practice- it is too easy to let things go for an easy life and while I agree you should pick your argument wisely, nevertheless I felt getting this particular ( albeit trivial) item out in the open actually cleared the air for me. The lady in question is quite a bit older than me (into her 80's but in full possession of the marbles so I can't put it down to forgetfulness!) and very opinionated - sometimes I do wonder if any of it is a wind-up as she can say some outrageously right wing things, but I suspect not(!) But the fact that she was so adamant that she knew was like a tiny little red rag to a bull to me !! As I said if the boot had been on the other foot, I would have been happy to "bow to her superior knowledge".
Standing up to people when they make unacceptable statements, especially in public can also take courage but reminds me of the poem which starts:
"First they came for the Communists, but I didn't speak out...."