Warning: Cannot modify header information - headers already sent
In my own case, I've found that this was due to modules that attempt to return a 404 by mistakenly calling drupal_not_found() instead of returning the MENU_NOT_FOUND constant. This causes drupal_deliver_page() to be called twice in a single page request.

Views is the biggest culprit right now; see https://drupal.org/node/1414068 (comment #16) for a patch that resolves it. You should grep through your modules directory for "drupal_not_found" to find other modules with this problem.