What are the common causes for a Zebra S4M printer to display a 'Ribbon In' warning even when a ribbon is physically present?

Common causes include improper ribbon loading (e.g., not fully seated, upside down, or incorrect threading path), a dirty or obstructed ribbon sensor that cannot accurately detect the ribbon's leader/trailer or body, using a non-standard or incompatible ribbon, or an incorrect printer configuration where the printer expects direct thermal media but a ribbon is present (or vice-versa).