I think errors like this are usually a muscle-mind error, not a misunderstanding error. I very frequently type "you're" when what I said in my head was "your". Sometimes I send/post before I notice the mistake.
The crazy one I do a lot (and usually notice after I can no longer edit) is "know" instead of "no", which is definitely a muscle-mind problem; it's not like I think "know" means "no".
You are right, I also forgot that it's a and not an.
Actually, I had a laugh. It's not every day you see grammatical corrections on HN. You earned my upvote
I also feel weird to correct people's spellings on HN, so you got my vote!
With everyone else defending misuse of language (LANGUAGE EVOLVES!) I am sure this is going to start becoming acceptable and correct.
you don't see official documents that Lo0k lyke dis! though, maybe informal but I doubt grammar errors will become acceptable in the near future
Using "they" instead of "he or she" is a grammatical error in many instances and it's considered acceptable.