In this question the author tried to paste a link to http://www.youtube.com/watch?v=ClxTi__1F0s. This produces the line:
1F0s">http://www.youtube.com/watch?v=ClxTi_1F0s
Instead of embedding the appropriate youtube video:
(I had to use the [x](x) syntax for embedding it, which is usually not required)
This seems to be related to the fact the URL has two underscores in it.
_<non-word-character>that's the problem. (Bug report on MSO) – Tim Stone Feb 11 '12 at 18:37