I am in the process of designing a WordPress theme, when I ran into the following problem. I designed the index.php page to my liking (using Blueprint and Underscore) and wanted to apply a similar style on my page.php.
I copy/pasted the same framework onto page.php and included a way of producing the Featured Image at the top of the page. The Admin Bar (the black bar at the top of the page when I am logged in) started reproducing itself in an odd fashion only on Pages. It also removes the header .svg I placed at the top of the page.
I don't think it was the Featured Image that caused the problem because going back to the original page.php & functions.php that Underscore provided doesn't make it go away.
NOTE: This problem shows itself on Chrome and Safari, but not Firefox. Firefox just hides the entire header (aside for the navigation bar). Weird.
The problematic page:
A page that works: