2.0.1 Error?

Occasionally when I view my site I see the following at the top of the page:

Warning: ob_start(): output handler ‘ob_gzhandler’ cannot be used after ‘URL-Rewriter’ in /home/omar/public_html/bullspeak/wp-includes/functions.php on line 829

Any ideas as to what this is or how to corect it? It does not always appear. I’m using Firefox 1.5.0.1 and am hosted by Verve Hosting.