It appears to happen haphazardly from this user's experience. It's happened on posting new messages, private messages, viewing forum posts, almost everywhere.
If it happens on a post, sometimes the post is completely hidden and only visible in the reply to post page where previous posts may be quoted. Other times it may not display on a thread as being the last post posted, and sometimes it may show up as a double post.
This thread has the hidden post effect on it... https://forum.atlas-games.com/t/1227-2t-the-dimicatio/7423/17
IF quote this particular post, the first one on the second page, and if the thread hasn't progressed too far before you look at it, you will note that there is a hidden post, preceding the post linked above that is identical, but just doesn't appear in the thread, unless viewed through the Topic Review section.
I've been getting the same error that JL quoted in the first post on this thread, but it is not happening consistently. It happened twice just now, once when I clicked my bookmark to the ArsMagica forum, and then after I logged in when it usually redirects to the board index. When I click the refresh button the pages load without error.