What could cause contact syncing errors on iPhone?
From your question, I think your setup involves syncing contacts with Google, a computer (Mac?), and an iPhone. Syncing with Google has been a pile of poo since Mountain Lion. For me, it kept duplicating contacts, bringing back deleted ones, and as you mentioned, swapping the contact information between contacts. 

Since neither Apple nor Google seem to care about fixing it, my solution is to use iCloud to sync contacts between the Mac and iPhone. (This seems to be Apple's preferred method, so it's the one in which bugs will likely be fixed the fastest.) I periodically upload a vCard of my contacts to Google. This is a lot less work than hunting down all the contacts that syncing with Google messes up.