I've seen a couple of threads over the weekend, where a poster has made an unsolicited comment on the grammar or spelling of another poster, where it had no relevance to the topic. (One where it was the OP's title that contained potentially a grammar error depending on what dialect they use, and the first response pointed it out - here if you want to see it: www.mumsnet.com/Talk/am_i_being_unreasonable/3235209-to-wonder-why-i-weren-t-orginally-invited?trending=1 ).
I think that is completely uncalled for and just gives pedants a bad name.
I try to be scrupulous in my own language and I am well aware of common errors. But while I may silently make a judgement when others make errors, I try not to even do that because we all make mistakes, and in fact I'm not convinced that errors correlate to laziness or lack of education - some people when typing fast in an informal setting just aren't that worried geting itall perfect as long as long as there meening is kleer.
In one thread, OP used "would of" and until another poster pointed it out, I confess I didn't even notice (life's too short to be focussing on these trivial mistakes). Update : I see that others criticised the latter poster and her post has now been deleted, so my view appears to coincide with the view of other MNetters and MNHQ (although some posters wrote in support of the "pedant" poster).
If I'm reading a newspaper article and see these kinds of errors, I'm not impressed, because they are professional writers with paid editors. But on MN Chat, who cares that much and is that much of an arse to draw attention to it publicly?
If the reader has been asked to proofread or it's an educational context, then fair enough, otherwise keep your pedantry to yourself or come on Pedants' Corner for a rant!