I saw this on Code Golf.SE:
So I thought, "Okay, I'll see an example!"
Apparently, it doesn't show, and this is the error:
Refused to display 'http://stackexchange.com/newsletters/newsletter?site=codegolf.stackexchange.com&suppressPromotion=true' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'.
When I go to the link it works. It appears to be a cross-domain issue, with Code Golf.SE trying to access http://stackexchange.com.