As a non-native speaker, I think there are some classes of errors like confusing homophones that native speakers are much more likely to make, e.g. "could of", their/there/they're, or even the above-mentioned its/it's. The latter two are covered very extensively in lessons, and "could of" is just so absurd to anyone who's had to study to learn the language that it doesn't even compute.

I've generally decided not to comment on people's spelling and grammar on HN because it doesn't really add to the discussion. I do sometimes make a point of italicising the correct spelling in my response if I have something substantial to add to the discussion :)