Seems like the 'Top Submitters' section on the left is a bit off, giving broken links to some users' profiles.  It's reproducible with users who have a space in their names (I found it trying to click on 'Unknown Horizons') - the URL is generated with the space in it, which leads to a User Not Found error.

Link is: http://opengameart.org/users/Unknown%20Horizons

Link should be: http://opengameart.org/users/unknown-horizons

Same thing shows up when clicking on user 'Ron AF Greve' - replacing spaces with dashes in the URL directs to the proper page.

-Moik