That depends...the function is in inc_functions.asp....if you like to bugcheck it and see if you can find a fix for the problem that would be great of course...other than that...don't use both url-tags in the same post for now...
I don't think this is a bug. Since I can't get to the topic that tomas posted, I believe that the error occured because users where enclosing a link, with [url][/url]. They don't need to do that because the forum already converts a link that starts with http://.