My brother goes by a contraction of his middle name, because my mam thought the names sounded odd the other way round.
I know a few people who go by their surname, or a shortening of it, to the point that their parents refer to them by that name: Vinnie is actually called David, and Jollie is actually Neil.
A friend at uni was called Sid by everybody, because when he first wrote his name as a kid it looked more like Sid than Simon.
I know someone else who got a passport in a name different to the one on his birth certificate, based on a note from his mum saying that was the name he used.