Going into a thread having read nothing more than the thread title and the OP, not reading whatever the OP links to or any replies, and then dropping a one-liner that was tired five topics ago. At least Ctrl+F through the thread and realize you are the tenth person to make a Kickstarter joke in a thread with Penny Arcade in the title/fifth person to make a PRISM or NSA joke in a Microsoft thread/whatever.
Going into a thread having read nothing more than the thread title and the OP, not reading whatever the OP links to or any replies, then asking a question that has been asked and answered three times a page for the past five pages.
Really it's just people who seem to think threads only really start when they show up. I get not having time to read everything in a huge thread, or a fast-moving thread, but at least skim the first and last couple of pages to get an idea of the conversation.