The Page 3 Effect is the phenomenon that on any online forum large enough, almost any thread will be derailed by page 3. Occurs most often in threads about touchy subjects, both serious (politics, AIDS, etc.) to stupid (Which class is overpowered in TF2, meme's, general flaming, etc.)
"Hey man, what happened to my thread?"
"Oh, it got locked because the Page 3 Effect kicked in."