Middle mouse button and Chrome
Chrome has just been updated to 28.0.1500.71 and opening a link in a new tab via the middle mouse button no longer works on Duolingo. The link will just open in the same tab. It works fine on every other website I've tried.
This appears to be a Google Chrome problem. You can read the bug report here: https://code.google.com/p/chromium/issues/detail?id=236835, but the basic idea is that there is a problem with opening urls that include a '#' in new tabs using the normal shortcut keys.
Apparently it was an issue in 27 too, but less visible (only occurs if you try to open the same link twice in a new tab).
The good news is that we plan to remove the #'s in the fairly near future, so it should return to normal functionality soon. For now, unfortunately, the only workaround appears to be to using right click, "Open Link in New Tab" or updating to a non stable version like 29 or Canary.
Thanks for the report!