How can many email, address and phone records suddenly disappear?
The reason proved to be that an admin user removed a Location Type. There is a clear warning if you try to do this via the UI.


  WARNING: Deleting this option will result in the loss of all location type records which use the option. This may mean the loss of a substantial amount of data, and the action cannot be undone. Do you want to continue?


Have asked about making the warning more specific in CRM-17273: Deleting Location Type should warn about loss of related contact data