If you try to post a new topic or reply, and you get an error message that says "Precondition Failed", this means that your message has been blocked by a security feature. This feature is intended to block spammers who use automated means to post advertising and other junk to the site. That said, the configuration is not perfect, and there are times when legitimate posts get blocked. When this happens, I may need to tweak the filters appropriately.
How to fix this problem:
Send an email to Forum Moderator Richard, and include the entire text that you were trying to post, including any HTML code. It may also be helpful to include your IP address -- you don't know your IP, go to http://www.whatismyip.com, that site will tell you what it is. Of course, also include your Nickname, email, and URL, that you were using when you received the error. With this information, I should be able to pinpoint the filter that caused the error, and should be able to tweak things to avoid the problem in future.