I pretty much always read the OP, but it seems a lot of people just read the title and go in and post. I will admit, I have been guilty of just skimming to OP because I had specific thoughts that I wanted to get down before I forgot them, and then in my haste I seem to forget to go back and make sure I'm posting about what the topic is actually about.
But plenty of times I've made topics where I put in big bold letters for everyone to see saying "Do not mention this! This isn't what the thread is about!" and then someone immediately mentions it and they're being completely serious. If I can find the thread I'm thinking, I'll edit it into this post.
EDIT: Here we go.
http://www.escapistmagazine.com/forums/read/9.197630-Left-4-Dead-3-Ideas#6406103
I put it in bold, italicized, and underlined it, wrote an entire paragraph explaining why I didn't want people to mention it and someone still goes and mentions it.