XWIKI-12718 introduced a parameter to output the page title along with its content. The problem is that sometimes the title is displayed even if we don't ask for it.
The root problem is that the rendercontent.vm defines a "local" $displayTitle variable to "cache" the value of the $request.outputTitle parameter. This variable can be overwritten by mistake when the page content is rendered.
This can easily be reproduce with the Blog Application. Put this in a page:
and then open that page in view mode with ?xpage=plain. The output should include only the blog post but it actually also includes the page title because the blogPostLayoutImage macro overwrites the $displayTitle variable.