RB fails to display a diff ("The patch to 'foo.c' didn't apply cleanly.") if the patch involves changes on the line containing $Header$ keyword. The reason is that SVNTool does not know about the $Header$ keyword and thus does not collapse it when preparing the original file for patching. $Header$ is standard SVN keyword, see here:

http://svnbook.red-bean.com/en/1.6/svn.advanced.props.special.keywords.html

./reviewboard/manage.py test