Putting a URL to a YouTube video displays the video in the content.
Putting square brackets around text, followed by a footnote URL turns the text into a link.
When the referenced link is a YouTube video, the link’s display text is removed, with the video replacing it.
Mark Gatiss played The Doctor in the spoof episode The Web of Caves alongside David Walliams.
NB: this bug seems to have been dealt with in meta.slant.co; but is still present on the main site.