This one hurts, Jason suggested to me to post this, and since Macadamian is an "egoless" programming shop - that I do adhere to - I do it even though I might be laughed at!
More seriously, when dealing with HTML, remember that each version of the specification has specific characteristics. And the application you are using (in my case Atlassian's Confluence), is validating against some version of the specification. Somehow I known this for years, having spent countless hours in the SOAP and WSDL specifications back in my Cognos days. Yet somehow I forgot, its easy to overlook these sorts of things sometimes.
Now the problem I encountered was around the closing tags. In XHTML you can do things like this but not in HTML4:
instead, in HTML4 you have to explicitely use the closing tag as shown below:
<iframe src=...> </iframe>
The consequence of this mistake was that all the code following the first form was not rendered by the Browser (I tested this in Firefox and Chrome). I could see the content driven by the iframe though.
So the morale of the story, read the manual and remember the specification! :)