I've had a couple of friends through the years who went by their middle names....however, their wives called them by their first names.
I was out once with Donna and mutual acquaintance when she referred to "Frank". Acquaintance said "Who's Frank? I thought your husband's name was John." Donna answered "Oh, Frank is the guy I sleep with."
Donna is hysterical and lightning quick!
OT but my DH has a name (let's pretend it's 'Mark') and ALL of his relatives refer to him as 'Marky' because his dad is also 'Mark'.
I loathe 'Marky' so I just call him 'Mark' and deal with the resultant confusion over 'which Mark?'
My boyfriend goes by his first middle name. His legal name is Firstname Middlename Odd Lastname. Apparently the Odd is a family tradition and it's Scandinavian (not sure which country it's from).
His dad's name is Firstname Odd Lastname, with the first and last names being the same. So BF isn't technically a Junior because he's got an extra name in there, but he's always gone by his middle name, except in legal documents. It really threw me for a loop when I was house sitting last spring and brought his mail in, I was half convinced it was all for the wrong person.
A friend of ours has a weird name situation. Her given name is (falsified) Hannah. She was married to a guy, took his last name, had a kid. Kid was very ill and passed, Hannah got divorced, but kept the last name because she wanted to remember her son and distance herself from her family. She went by Hannah during all of this. Fast forward, she gets engaged again, to another guy with a different last name. They get married, but now she doesn't want to go by Hannah anymore, she wants to go by a different name entirely, which is a completely made up name. (She's admitted it's made up, I'm not just saying that because I'm not familiar with it.) So on Facebook her name is MadeUp Hannah MaidenName ExLastName LastName. So anyone who knows her as MadeUp finds it really confusing when half of her friends refer to her as MadeUp and Hannah in the same conversation.