"CONTAINED_STUFF" is a bit of a special string within ecore.
The container system that's used to generate the nodelets and things on every page, regardless of the page's content, replaces the first instance of the string 'CONTAINED_STUFF' in the container HTML with what's actually contained... normally, that's the writeup.
However, this process is applied recursively, beginning with the containing container for the htmlpage for the requested note, and following each container's 'parent container'. Usually, htmlpages specify the general container as their parent. This in turn is contained within the main container (or a variation as specified by a theme), which is in turn contained within the stdcontainer, which has no parent.
So, first stdcontainer is evaluated. Then main container is evaluated, and the text 'CONTAINED_STUFF' in the result of stdcontainer is replaced with the result of main container. This is repeated for main container and general container.
If the text 'CONTAINED_STUFF' appears more than once in the result of the container, the first one is usually the only one substituted. Usually 'CONTAINED_STUFF' is written into the source for the container. However, if it appears more than once, for example because it's a node title (which the main container will show), the wrong instance may get the contained text.
With hilarious consequences. Until someone fixes the bug.
Edit, 9th May 2009: the fun is over. It is fixed. *sniff*.