Sometimes the auto-generated URL tag shows up in the finished post. I've had that happen too; IIRC it's when you already had a URL tag and then you try to use the automatic one, which adds a nested tag or something that UBB can't parse.
Just go back and edit out the resulting mess, and replace it with:
<font class="small">Code:</font><hr /><pre>
your text </pre><hr />
(I'm glad I got the code formatting tag idea from illunious's post above!)