It's exactly as Illori suggests.
It sporatically works across my site.
some specific sites work and sometimes don't.
sometimes deviantart url's are correctly proxied, sometimes they 404.
same happens with tinyurl, tumblr, tinypic, various wikia pages, imgur still occasionally does it.
The images that don't work on my site, nor display, however they work here after 2-3 refreshes.