We all have (or had) that friend who so cordially points out our every flaw. We all have those classmates who you just seem to rub the wrong way. The important thing is not to let these people bring you down with their insistent pointing out of errors that do not pertain to them or anyone else. Minor flaws that are merely human. As long as they hurt no one, it shouldn't be of interest or of issue to others. Everyone has them, even the know-it-alls that content themselves criticizing When encountering these people, remember those who would happily defend you. That friend who would stand up and say for you, "that was uncalled for." We all have flaws, and there will always be someone pointing them out like it is all they are meant to do. Just remember those who would raise their voice in your favor, because they probably know you better and are a better judge. Blocking out the downers and bringing in happy people is one of the keys to a happier existence.
Now, don't get me wrong, constructive criticism is good in my book. We all have something to learn that can only be learnt if pointed out. But just strait up criticism is usually unnecessary.