I can’t reproduce this bug with any regularity, but:
(Running IE11 on Win 7)
If I enter a thread with more than ~20 posts and read the latest post, then scroll up past the point where the page needs to load more older posts, and then expand the previous reply history to one of those older posts, the following happens when I go back to the bottom (generally by pushing the “End” key):
Note that some of the icons for the users are being pushed left to the right edge of the previous line, and that the bottom toolbar has been pushed to the right side of the screen.
I’ve been able to break it more than this (the actual post contents ending up below the reply bar), but do not have a screenshot.
It’s only been happening this week, as far as I’ve noticed.