AJAX Reply SQL Error 1064
It's still happening.
The page refresh on AJAX Quick Reply that then displays the double post error redirect.
This error is even more obscure than the Auto Link one. We know why it happens: $threadinfo['threadid'] turns NULL between newpost_complete and newreply_post_ajax. The culprit, however, is elusive, because we made sure for the stable release to only replace values with non-NULL values.
The main problem with being able to locate the exact source of the problem is that the error does not occur consistently, but seemingly randomly. One can use AJAX Quick Reply many times without a problem.