I just tried to embed a Youtube video in this post, but instead of Youtubifying it, it just prints the URL. However, if I cut and paste the video-code of another video into my markup, the video works.

[http://www.youtube.com/watch?v=2ImZTwYwCug]
produces

whereas

[http://www.youtube.com/watch?v=1CGO5imNLPc]
produces
http://www.youtube.com/watch?v=1CGO5imNLPc

I tried it on a few random Youtube videos and they were embeddable.

AE.

Some YouTube videos cannot be embedded. This is a choice the uploader makes.

For a moment, I had a theory that the video could not be viewed in the USA, and hence could not be embedded by the a.cc server.