On clean drupal 8 installation
- go to Structure > Blocks (admin/structure/block)
- click on Seven tab (admin/structure/block/list/block_plugin_ui%253Aseven)
- click on "Demonstrate block regions (Seven)" (admin/structure/block/demo/seven)
get page with very small content
after clicking on 'Exit block region demonstration' (admin/structure/block/list/seven)
get a 'Page not found' error
Originally reported https://drupal.org/node/1987636#comment-7594383