There's an annoying behavior I've recently started noticing (say, over the last year), and I wonder where it came from. There's an example at Megan McArdle's comment section. The commenter takes a previously-published line, makes a meaningful change to it, and then closes with something liked "fixed that for you," as if the original commentor or poster had made an obvious mistake. No reason supporting the change is offered, it's just a slap in the face, and one I can't even tell is correct.
It's self-serving, doesn't add to the conversation, and gives me no reason to respect anything the person making the comment has to say. Don't do it.