returning ghost matches

[mister tapeworm]

I'm using the following query:

SELECT `id`, `title`, `body` FROM `threads` WHERE `id` IN (SELECT `id` FROM `threads` WHERE MATCH(`title`, `body`) AGAINST ('http://www.websitethatdoesnotexist.com'; IN BOOLEAN MODE) UNION SELECT `threadid` FROM `posts` WHERE MATCH(`body`) AGAINST ('http://www.websitethatdoesnotexist.com'; IN BOOLEAN MODE) AND `valid` = 'Y')

query works fine, in all ways possible EXCEPT, for some reason searching for a web address (like the one above) which is not referenced anywhere in my database causes the query to return a large number of results when there should be none... why?