On sites that allow you to post an answer without logging in I get a horizontal scrollbar even when the site has a responsive design, probably due to the sign up/login boxes:
Example links to sites where this happens:
This happens at all browser widths under a certain number, although the scrollbar changes sizes.
It doesn't matter if I'm signed into an account on another site or not.
As you can see in the screenshot, the left nav is enabled but you can't see it because I scrolled to the right. On the right is just blank space, since all content (other than the sign in/login boxes) are the right width for the page were it not to have a scrollbar.
Happens in Chrome and Safari on Mac.