Recently, when I tried to make new post in old theme, I seen exactly the same problem now and again: after I am posting anything, I don't returning to theme, but receiving "error, try again to post later" - but if I'll do that, I'll have double post, as old one was posted already!
It is in Opera, as in home, as at work.
Editing old post is immune to that. Just as posting new theme. Only new post in old theme is prone to that (or maybe it's exact forum bug, I dunno).
Yes, I know that I can fill bug report form... but I have a very little trust in this reports. Can anyone send personal letter?
---edit---
Hmm, I cannot reproduce this error willingly, it seems. :| Ok, forget it then. Maybe it's time for answer or something similarly obscure...
No way to tell for sure then.