"OP: I would like to buy a lovely perfume for my DSis who has just finished chemo. What's you're favourite?
Poster 1: What a nice thing to do! I like Chanel 19
Poster 2: Actually it's "your favourite"
Poster 3: Perfume is so personal. Do you know whether she likes floral/citrus/woody family? Here's a link to a helpful website.
Poster 4: I can't believe you were so insensitive Poster 2! OP is trying to do something nice for her sick sister and you're bashing her grammar!"
If it was left there then that would be fine. Poster 2 does deserve a telling off, or maybe two. But 500 posts later we are still alternating between odd-numbered posts where people make helpful suggestions about perfumes, and even numbered posts where people attack Poster 2's general poor character.
Can I please beg of you.
If you start to read a long thread, get three posts in and see a poster behaving very badly in a way which has nothing to do with the subject of the thread. Please please please don't immediately assume that you were the only person to spot this poor behaviour and you must cease reading immediately to reprimand them. Have a quick check down the thread to see whether fifty people have already put the boot in in increasingly harsh terms. Yes maybe your devastatingly worded fifty first reprimand will be the one which finally shows her the error of her ways, but that's not the way to bet.
And in the meantime the OP's helpful and lovely thread about perfume and sisterly love has become a depressing morass of people bitching about apostrophes.