I have come across one strange thing in the contacts. I noticed that one contact has two phone numbers, one is an old number that is not valid any more. I wanted to delete this number but it did not show at all in the edit contact screen. But in the normal contact screen both numbers are shown, both as Mobile number. So I changed the correct number to be Home number, and then both numbers are shown in the edit contact. But the unwanted number is grayed out and can not be edited.
I wondered for a long time where this old number came from, and finally understood that it comes from his facebook profile.

So there were two problems:
- If there were two phone numbers, both marked as Mobile number, only one was visible in the edit contact.
- You can not remove a number that comes from contact's facebook profile. And there is no indication that the number comes from facebook profile.