Width of teaser can be higher than width of white page container in example client.
It appears CSS should be adjusted to stay within the white box on page. A starting point could be main.css:433 of example client.
Steps to reproduce:
Install example client with example modules
Go to Content -> Articles
Click on category Main Navigation -> Features -> Content -> Teaser
Go to editor tab for article Teaser
Go to manual tab of Teaser settings
Include 4 articles