"Notice: this variable may not be empty: $_POST["thread_sort_mode"]"
when attempting to post a reply to a forum post when they preview the intended post.
This is s known bug 2991 in TW 4.x. See "No save after preview of reply by non-admins" http://dev.tikiwiki.org/tiki-view_tracker_item.php?itemId=2991 which perfectly describes the issue we're seeing.
The patch which we previously applied to v4.2 does not appear to be effective for v4.3
(Was Issue #11 - Forum Reply Posts Broken initially reported 13 Oct 2010 from migration v4.2 to v4.3)