We upgraded to ACT! Premium 2013 and then decided to go to ACT! for Web (which required an uninstall and reinstall) and then had to go back to straight ACT! Premium (another uninstall/reinstall) after having conflicts with other local websites.
Somewhere in the transitions, some of our Contact data becamse missing. Right now, it seems to be random email addresses (the contact is still there, the email field is blank). There doesn't seem to be a rhyme or reason to the missing data. I tried reindexing and repairing the database but that hasn't made a difference.