I really hate it when I do something like send a letter or an email, with my name in the address and the signature, and someone will reply with "Dear Kathy". It's so presumptuous, but we do that with diminutives in our society. If I were Robert, I'm sure I'd be getting "Dear Bob" (though probably not "Dear Bobby"). I think it is kind of rude. It is one thing if someone introduces themselves to you that way, but unless you are at a certain degree of familiarity and/or intimacy it is pretty ill-mannered to shorten someone's name. Yet it happens all the time.
Personally, given I have always hated my given name, I find it really gets my back up when people I'm dealing with in an official capacity feel that seeing my name on some ID or on a computer screen gives the=m the right to call me by it. Nowadays the only people that actually call me by it are my parents, but none of my friends get to use it. I've been Ghoti for nigh on 15 years now, and dammit, that's what people can call me if they want me to acknowledge it.
Sometimes these sorts of mistakes are extremely useful. When someone calls and asks for Mr. Bull or Mrs. Shetterly, we immediately say, "What are you selling?" Sometimes we go straight to: "There's no one by that name here. Please take us off your calling list."
Likewise. Given the phone is in the Housemate's name, if someone calls asking for Mr or Mrs X, then I just tell them "Sorry, no-one here by that name. You must have the wrong number."
I've had the opposite problem in terms of names - I hate my full first name, but I've had people who, even after I've worked with them for years, insist on using it no matter how many times I ask them not to. The two worst offenders were both Brits and old enough to be my parents, so I'm not sure if it was a generational or cultural thing or both.
I've avoided that by just not giving people my first name unless I absolutely have to. At some point I'll have to get around to legally changing my name, so I can stop having to give it out ever. =P
People insisting on using a name or form of a name that you have repeatedly asked them not to seems the height of rudeness to my mind.