Just for you:
A while back there was a stage where several people on forum would attempt to 'derail' threads by posting pictures of trains that have been derailed. From such an image of a wrecked train signifies a thread is no longer on topic, it has been derailed.
There was also the great pyramid quote, where each post quoted the one before until the quote tree took over the page. Unfortunately the thread never managed a pyramid great enough for it to collapse under it's own weight and turn into a black hole.