One of the most negative habits is in my opinion the failure to read a comment charitably (to make an effort to interpret it in the best possible light). Instead, people often tend to misread or miserunderstand what's being said, only to use the opportunity to write a "correction" based on that false impression. A worse flavor of the same problem is a misunderstanding that is then used to justify outrage or personal…
And the saddest part is not seeing the interlocutor full of himself and completely ignoring the fact that, by not using this principle, he makes his own argument weaker (showing that his intelligence didn't reached those heights).
The saddest part is the public false victory because depending on the peers (audience), he will likely be perceived as in a stronger position when the opposite is true.