Had this happen to me on Chrome on my Android device earlier, but ignored it. Now it's happened on Chrome on my Win7 machine too. See screenshots:
Sorry the pictures are so huge, but its the only way I could fit it in. As you can see the first post looks like a normal thread. The second one is scrolled down and after a large black gap the first post repeats (as does the rest of the page of posts afterwards).
(For the record, happened on this thread: http://www.giantbomb.com/sleeping-dogs/3030-29441/forums/can-you-beat-the-new-sleeping-dogs-challenges-1433001/#2)