Nicknames
This friend of the family has this irritating habit of calling my eldest "Bobby." We've never called him that, nor does anyone else in the family. We called him Bob when he was a baby (still do), though he prefers Robert. For a brief time in kindergarten, he had a dalliance with "Robbie," though that didn't last.
Whenever he says it, I gently correct him by responding with Robert's proper name. But he doesn't seem to get it.
It'll go something like, "How's Bobby doing in school?"
And I'll say, "Robert's doing fine, thanks."
At some point I reach a point where I want to scream, "His name is Robert, what are you, fucking retarded?" but I always dial it back.
I feel pretty strongl that there's an etiquette you have to follow with nicknames that's pretty simple. Unless some addresses himself or herself with a nickname, you shouldn't use one -- unless there's a nickname that's tacitly accepted by the whole family or group.