When I was young I couldn't say my whole last name so I shortened it to just the last two syllables - Eppe (pronounded Eppie). My Aunt used to call me Susie Eppie. To this day she is the ONLY person ever allowed to get away with calling me "Susie" anything more than once.
Strangely enough, my family all call me Susan and my friends all call me Sue. It was never planned, it just happened that way. I've never heard my best friend call me Susan. Never.