If you fiddle with {{DISPLAYTITLE}}, e.g. to hide the main page, then the Blueprint sidebar displays the HTML text of the CSS as the title.

To reproduce, create a page like http://livingstyleguide.wmflabs.org/wiki/Displaytitle_bug

The fix is easy, just use some other key from QuickTemplate's data that shows a cleaned-up title. For devhub I used {{titletext}} instead of {{title}}, which also drops the namespace prefix.