On my iPhone, I have a guy, let’s call him “Laurent”. And I have his email address there. It looks like this:
Laurent
home: boo@baz.it
Then later I entered a contact on my Mac with full details (phone number, address, etc). I tried to sync by connecting the USB cable. I was naive. On Mavericks and iOS7, you can’t trigger a sync like this. It has to go through iCloud. Boy, do I feel secure giving all my contact data to Apple and the NSA, but that’s another topic. On the Mac, it looks like this:
Laurent
work: foo@bar.com
home: boo@baz.it
So now, after a few seconds, I see that it “sync’ed” something, because now on the Mac, I have the same email address twice. I.e. it looks like this:
Laurent
work: foo@bar.com
home: boo@baz.it
home:boo@baz.it.
Needless to say, I suppress the second address, right? You have to wonder why Contacts didn’t do it by itself. I guess it’s for my own good. So now on the Mac, I’m back to:
Laurent
work: foo@bar.com
home: boo@baz.it
Back to the iPhone, hoping that this time, it had sync’d the entire vCard. But no, it’s waaaay to smart for something that simple. Now, on the iPhone, it removed the email address, but did not add the rest. So my card looks like this:
Laurent
That’s a joke, right?
Grenouille Bouillie 
The Mac OS X childhood diseases get weirder and weirder for each new version. I always wait for the initial round of patches before upgrading. I wish they would spend more time on each system instead of this increased release rate, but hey, at least we get it for free. (or for €1000 depending on how you view it)