For some reason the other post didn't even get anywhere after pressing "Post" button.
This sometimes happens, I suspect it's something with the session check. I usually go back in history copy the contents and start over (load the board and create a new thread again).
You have been banned for sp... a... min...ng.. o wait, it's a legit user.
As An_dz said, probably your login session expired and the forum refused to accept your post.
To avoid this, log out and log in back with a longer session length. Use "Forever" only on your private computer.
This might happen not only because you have set a low session timeout but also because the forum cookie is always being updated to maintain your session and make sure you're doing the actions. If you block cookies in your browser you can't post.
But thinking now I guess it must be a desync with the server or a bad sent request.
PLUS: it just happened while trying to post this.