Repro steps:

• Enable "Hide placeholders of blocked elements" setting.
• Globally block 3rd-party frames using advanced user mode.
• Go to http://gawker.com/watch-the-most-deeply-uncomfortable-moments-of-the-ted-1746198836.
• The embedded Youtube video at the top of the article is properly blocked, but the corresponding DOM element is not collapsed.

The src property for the embedded iframe is /ajax/inset/iframe?id=youtube-video-KqpHTFJO4mE&start=0, which means that the iframe appears to be 1st-party.

However there is redirection server-side which causes the effective URL for the iframe to be https://www.youtube.com/embed/KqpHTFJO4mE?wmode=transparent&rel=0&autohide=1&enablejsapi=1&showinfo=0&start=0, which is properly blocked.

However the DOM element collapser uses the original src attribute to find out whether collapsing must take place, hence it fails to collapse the iframe which was blocked using redirected URL.