We have a list that serves as an fairly large contacts list. For
simplicity's sake, we used a Person field to identify the contact so we
could use presence. I created a workflow to copy the person field to
the title field, which worked like a charm for the dozen or so I
tested. However, after migrating several hundred contacts into the
list, we found about 25 where the workflow had errored out.
Investigating it uncovered that all 25 people picked for those contacts
were no longer with the company, although they appeared in the
SharePoint person lookup. Once I removed them from the All People list
in the site collection, they no longer appeared in the SharePoint
lookup.