We are having an issue with JoomSEF creating some bad URLs and some are resulting in error pages (eg: 500 error). The only way we have found to resolve this issue is to delete the SEF URL from JoomSEF, but we need to find a more permanent solution to prevent JoomSEF from creating these URLs. A screen of what we are experiencing is attached. Does anyone know how to fix this?

http://http://imageshack.us/photo/my-images/94/joomseferror2.jpg/

Hi,
Maybe the highlight variable is causing your problems. Open Content extension parameters from JoomSEF's Manage Extensions page and there on the Common tab add "highlight" to Custom non-SEF variables field. Then delete SEF URLs that contain the highlight variable.