Visiting this page I see this advertisement in the right-side sidebar:
I'm not sure if this is because I'm visiting from a page with an https URL or if it's something caused by the https-everywhere extension I recently installed.
I am using
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:27.0) Gecko/20100101 Firefox/27.0.
I understand that SE is still working on support for HTTPS and that the support is so far incomplete. This is a bug report, not a request for help.
ADDED 2014-02-24: The URL of the broken image is, as predicted below by user 127.0.9.6, is http://stack-exchange-dynamic-ads.herokuapp.com/bounty.png