You don't need to give lexcorvus a hard time. Isn't it evident from his/her personality that he/she has enough of those already?

A lot of people know full well the difference between "its" and "it's", but dash off HN comments because they have something contribute but not a lot of time. I've been known to forget whole verbs when writing out my comments, for example. That doesn't mean that they're fated to a lifetime of always getting it wrong: the parent comment you corrected used "it's" correctly once and incorrectly twice, and uses "its" correctly in his comment history.

The other commenters are asking you to please consider carelessness rather incompetence as a motive, and to see the forest for the trees. If everybody corrected grammar in the comments, we'd quickly see the site switch from substantive discussion to useless pedantry. Don't be that guy.