Since we're talking posts as well, I'll nominate that thing people do where there are already active posts on a topic, but they'll pretend they didn't see them and start another about the exact same thing... presumably because they see the subject is getting a lot of engagement and want to reposition it with themselves as the center of attention.

On a case-by-case basis I can't prove that's what any given individual is doing, but it happens way too often and sometimes it's such an out-of-the-blue topic that I have a hard time believing the 2nd guy thought of it independently without having seen the other post. Plus you can often tell it's about them getting attention because they're usually some thread-sitting mfers lol