The reason you don't do this is there is a real possibility they are willing to pay $X+25%, so now you've just anchored the discussion around $X and you'll like

The reason you don't do this is there is a real possibility they are willing to pay $X+25%, so now you've just anchored the discussion around $X and you'll likely not see an offer for more unless you've really low balled yourself and the employer has set salary bands. Also definitely do not follow up with the '...but i'm willing to negotiate' bit.