Error attempting to publish non-existent node revision

We encountered an issue when running cron which produced the errors pasted at the bottom of this issue. I figured out that there was a non-existent node revision scheduled to publish. Once I cleared the applicable entries from the state_flow_schedule and queue tables, the error went away.

So the question has to be, how was a non-existent node revision scheduled? My guess is that either we scheduled the revision, then deleted it, and the database didn't get updated properly...or something similar with one version of the node getting reverted and not updating the schedule table properly when vids got shifted around.

Here're the errors:

Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Undefined index: headline in field_extra_fields_get_display() (line 693 of /docroot/modules/field/field.module).
Notice: Trying to get property of non-object in state_flow_schedule_process_item() (line 174 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow_schedule/state_flow_schedule.module).
Notice: Trying to get property of non-object in state_flow_schedule_process_item() (line 174 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow_schedule/state_flow_schedule.module).
Notice: Trying to get property of non-object in state_flow_load_state_machine() (line 409 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow/state_flow.module).
Notice: Trying to get property of non-object in state_flow_load_state_machine() (line 420 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow/state_flow.module).
Notice: Trying to get property of non-object in state_flow_load_state_machine() (line 423 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow/state_flow.module).
Notice: Trying to get property of non-object in StateFlow->persist() (line 200 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow/plugins/state_flow.inc).
Notice: Trying to get property of non-object in StateFlow->persist() (line 201 of /docroot/profiles/all/modules/contrib/state_machine/modules/state_flow/plugins/state_flow.inc).
PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '0' for key 'PRIMARY': INSERT INTO {node_revision_states} (vid, nid, state, status, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4); Array ( [:db_insert_placeholder_0] => 0 [:db_insert_placeholder_1] => 0 [:db_insert_placeholder_2] => published [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => 1355168858 ) in drupal_write_record() (line 7108 of /docroot/includes/common.inc).
Status message Attempting to publish nid:, vid: