If there is no http:// at the beginning, the current website's domain is used instead.

Example:
[a href="www.youtube.com/watch?v=NGI0c5YASNg"]Like this link[/a]
Like this link becomes http://www.dreamwidth.org/www.youtube.com/watch?v=NGI0c5YASNg

but
[a href="http://www.youtube.com/watch?v=NGI0c5YASNg"]This one[/a]
This one becomes http://www.youtube.com/watch?v=NGI0c5YASNg

I don't know if this is exactly what you did though.