This is a very small thing, but I deal in very small things professionally and so I hope you will take my suggestion into consideration:
When replying to a thread, there is an unexpected hierarchy of the buttons which does not put the most obvious/desired button as the foremost.
When one is replying to a thread, you get the usual text box where you type your reply, and then really your only interest is "post." Sometimes you want to "preview", lord knows I am obsessive about getting it just right, but generally, you want to "post."
Fooling around with the page has shown me that there is a distinction between a "new post" meaning, the "new post" button which is at the bottom of an already existing thread, which is actually a reply to the opening post of the entire thread, and a "reply", which is a reply to a very specific "new post" in regards to the OP. However, conceptually, these things are not clearly demarkated, and so, as a new user, I clicked "reply" to a post (1), got the text box and filled in my scintillating words of wisdom (2), went right to the larger, red text "new post" button (3), and skipped past the smaller, black text "post" buttons (4), resulting in the nearly indecipherable error of "You have an active editor somewhere already and it is not possible to edit multiple elements at once."
Being new to the site, but not at all new to wikis nor forums, this made me think I had another tab open, perhaps? But it did not indicate to me that what I actually wanted was the unassuming "post" button just below the text entry box (once again, 4).
The indentation of these buttons (where the more leftmost something is, the more scannable it is in general), the relative size of the buttons (denoting relative importance, ie what I want most), and the red text on one button versus the black on the "actual" button, all gave visual signals that I wanted a button I did not in fact want.
Now, if you want my genuine opinion, I feel that the entire distinction between a "reply" and a "new post" is a false one, at least from a user's standpoint, but I digress and would instead suggest that the simple fix would be to hide the "new post" button when one is replying to another post in a thread. It is interesting to note that the "new post" button is hidden when I am making a "new post", but not when I am replying, which seems desired to me for all replies.
Of course, this is something that, once learned, you'd hope people would retain. However, studies have shown that new users, or old ones over periods of absence, will lose these "learned expectations" which defy broader usability expectations and repeat them several times before relearning the actual behavior. However, people who use the site every day will recall the proper path to success, so it's up to you if this is worth fixing if there are larger errors at hand.
Ok, now that you have read a whole page of text regarding the size of buttons on a forum, I bid you goodnight.