if it's just you and me, and I ask "oh have you seen Citizen Kane" and you're like "No," it would be a pretty douchey thing for me to go "IT WAS HIS SLED LOL."
But if you're on a forum, interrupting others who HAVE ALL READ THE BOOK and stop them from having a conversation that at no point included you, then yeah, I get a little irritated.
And I am perfectly willing to admit that the timeline is a gray area. How long does someone have to read Harry Potter, a week? A month? Two years?
My main point is: I don't mind being respectful of people who haven't read/seen something, but you have to be respectful of the people who have.