Have you ever..

[Answerprancer]

...accidentally called a person by a nickname they didn't know they had?

I did it once - the person in question owed me money - he is known as "Sunbird".
I spent ages chasing him for this money and in the meantime I was deliberately referring to him behind his back as "Sunburn" then one day I phoned him and said "is this Sunburn?"
He sort of went "Yuuuurrsss?" Most embarrassing, then again - the guy is a kundt.

[boxtops]

My mum went to a parents' night and called our physics teacher by his nickname instead of Mr *** - oops.

[lardhelmet]

Yes - I once called the woman in the office after whom I'm named - Lardhelmet! Needless to say she didn't realise I was talking to her! lol