I've run into it quite often, had a whole grammatical debate over it once because the guy perceived not being accepted as an attack.
Grammar and spelling can annoy me at times.
I've seen a character, lets call her Billie, and another player kept writing "Billy" as the name. Like for fucks sake... why write a character sheet when people don't read or refer back to?
As for grammar I can understand minor things like incorrect sentence structure if it's still a well written post and that one section was written oddly. I probably have the same thing and I know I latch onto words and repeat them without thinking. What gets me is the failed use of capitals, punctuation, and failing to enclose a whole sentence in a defined way (eg: "They went to the shops to buy." — Um... Buy what?).
Naturally I would think that people who join a writing forum would come here to also better their skills, but as you pointed out @Gowi, some people get the shits when you correct them so that they can improve.