Steps to replicate :

1. Install Confluence 6.0.1
2. Login to Confluence
3. Create a space
4. Navigate to Space Tools >> Look and Feel >> Sidebar, header and footer
5.  Add 'x' into the header field and click Save
6.  Navigate back to the space home page and click edit
7.  Click Preview

Current result : The preview UI is broken as per the screenshot below :

Expected result : The preview UI should be displayed properly.

Workaround

Adding to Global CSS ( > General Configuration > Stylesheet) can help

body.contenteditor #custom-content-header {display:none;}
body.contenteditor #custom-content-footer {display:none;}