Not sure how best to describe this, but basically, if I have a contact with multiple instances of the same detail type (eg: multiple work phone numbers) then when I edit another contact and then that contact weird things happen and some of my changes get lost.

Steps to reproduce :
(note this is done using the local address book,
also, I use "work phone number" but I assume the same is true for other types of numbers)

1. Have 2 contacts, contact A has one work phone number, contact B has 2 work phone numbers.
2. Edit work phone number of contact A.
3. Edit the second work phone number of contact B.
4. Now return to contact A, the work phone number of contact A is now the same as the first work phone number of contact B.
5. Not always, but often if you go back to contact B, the changes which were made to the second work phone number are lost. Other times the second phone number will just be completely missing.
6. Occasionally an info bar will pop up that says "error setting property phone-number...." or something about an "unknown error", I don't have time to read it before it goes away.

Sometimes this has succeeded, but it fails most of the time.

Variations of this fail too, eg: editing contact B first and then contact A.

Often a similar bug will occur if I try to add a work phone number to a contact which already has one and the detail is usually never added. In this case I also receive the following error in my terminal:
"(gnome-contacts:24063): folks-WARNING **: phone-details.vala:72: Empty phone number passed to PhoneFieldDetails."

A workaround to these issues is to close and re-open gnome-contacts after any edit operation.