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.
Might not hurt to check the logs starting about 9:44am EST, today, 12/6/2012, the fora became all but unusable for 30-45 minutes.
I didn't realise this had been posted here (so posted the same on another thread). But I'm getting the same error message about 2 our of every 3 times I come to the forum or refresh any page.
Any thoughts?
Our webmaster reports that the updates she just did, in response to complaints that the forums were having a lot of problems, went well. Please let me know if you're still having trouble. Thanks.
I noticed that there were updates this morning, when I clicked on the Atlas logo to be taken to the blog.atlas-games.com page didn't work. 
So far so good.
Since my last response I've had two repeats of the error I quoted in the OP. So it's less, but not completely solved.
Just had a 20 minute period of attempting to post a message, and being greeted by the "error" page I indicated in the OP.
I've let our webmaster know.
Anyone else getting sporadic 500 Internal Server Errors? I've seen it in the last couple of days, but it just happens briefly and then the site seems to work ok.