The problem is when editing a post using the overlay editor, with IE10 (and likely other versions, which I no longer have) as the browser. You can edit an existing post one time. But, go to edit it a second or third time, and the editing area cannot receive focus, and you cannot edit the message. The buttons below the post do work, however. If you refresh the page, you can then edit the post. And again, you can only edit it once.
With Chrome, I am able to edit a post a dozen times in a row without any ill effect.
After searching here and browsing to no avail, I could not find another reported bug (unless someone has submitted it with an unclear thread title). Apologies if it's a duplicate.
For XF staff: I have created a login and a super admin account, plus a direct FTP login, if you need to access my unmodified test forum. Reply here if you need access and I'll send details privately.