In some rare circumstances you may end up with recurring contribution records that do not belong to a contact. If this happens, you will get the following error message when upgrading to 4.3.8 or later:
To fix this problem, you can run the following sql script to delete the bad data. We strongly recommend that you try this first on a TEST copy of your database, and that you make a backup prior to running it against your production database.
Further reading: CRM-13446