There is an issue with IE as well where pasting in junk HTML generated by MS Word can break the page rendering at the start of a post (rather than at an arbitrary page length as the OP noted, regardless where that winds up), but it's easily remedied by editing the post or not copy/pasting directly from Word.
In the case of this issue it tends to be replies by particular users who frequently do that that are at issue, rather than the thread just being really long. If you see one of these such threads, report it and a moderator will edit the offending post.