I've seen the same type of issue and have figured out how to reproduce at-will (and also how to now avoid). If you enter something into the "search" field and then attempt to add a new contact.. some short period of time later the edit of that new contact will abort and you will have some other contact already in your database open in edit mode. You can at this point click "done" without changing anything for the existing contact. The partially completed new contact might not show up in your contacts for a short while.. or you can close contacts and re-open to hopefully now find that new partially completed (because interrupted) new contact.. edit and complete the input of the new contact.
The bug has to do with having an active search first and then trying to add a new contact. Very common thing.. have new contact info for Joe Bug.. search first to see if already in contacts.. don't see him in your search results. Start adding new and then *wham* it aborts while in the middle of typing..
If you do not search or if you clear the search first before entering the new contact, it will not do this anymore.
So, there's the bug.. please now Apple - go fix it.
Now.. I've seen another bug.. which I can't replicate at-will yet.. sometimes I begin entering new info (like a name) into a new contact and the name is displayed backwards!! I'm not typing it backwards.. Jane Bug shows up as enaJ guB. Very weird...