It's hard for me to be subjective because this teacher (not my own DC teacher) is initiating a new activity for the whole school that seems to be badly organised and is causing a hassle.
So I was already kind of annoyed with them already, but getting a parentmail with a correction to their previous mailing, saying "In my last letter x should of been y" has pushed me over the edge.
I get on well with the school, have a good relationship with them, and am not known for kicking up a fuss. Do I say anything? Although I am a pedant, I'm not a total stickler. I realise mistakes with punctuation etc. can happen and I generally let them go – but "should of" suggests ongoing ignorance, not a slip-up.