A #2 masterpin can fall out when the key used to operate a cross-keyed cylinder is partially inserted or partially removed prior to completing the key rotation.  This issue is typically seen in a cross-keyed cylinder with a stack of #2 masterpins in the last chamber.  Since multiple chambers are empty and usually with 2 chambers have masterpins a partially inserted (or removed) key can create a shear line allowing the key to rotate.  Depending on the key used a #2 masterpin can be pushed into the "bible" and with a partially inserted or removed key the end of the key can't keep the #2 masterpin in the bible thus it falls out.