We recently had a spate of fail mail on omnimail_recipient_process_unsubscribes - it turned out to relate to an attempt to add data to a contact that had been hard deleted - but the contact_id was still wrong in the civicrm_mailing_provider_data table

It looks like the contact was merged on 2024-09-21 13:35:06 - I can't recall when we started updating the contact_id on that table when merges happen but I am sure it was well before that so maybe this was a merge that crashed? We can investigate by querying rows in that table with contact IDs not in the civicrm_contact table & attempting to fix them. We may be able to monitor them like we do contributions against deleted contacts going forwards