I find that I *continually* have to edit my posts because I've misused there, their and they're. I know full well the difference, but I will always type "there" without thinking, making me edit the post and changing it. Normally it's not until I've hit "Post Message" until I realise the mistake I've made. And even then there will be one that I miss. I don't tend to make the other mistakes (as often, that is.) Though I do use these a lot: !!!.
I think it is down to the informality and relaxed atmosphere of the forum. I don't type a post like I would an email (unless to friends), an essay or a report. I think it's easy to make these mistakes since you're not thinking about it as much.
And linked to this, which grammar/spelling/punctuation mistakes irritate you most? I'm not particularly bothered about errors (it's only human!), but I annoy myself that I have to always edit there, their and they're.
(Checks post for no mistakes. Look's good to me!)