My first name is Jonathan, I generally get referred to as (int al) Jon, Jonny, Jo, or John (bloody silent letters).
As it turns out, until I was 20 I thought my name was spelt Jonathon. I got a copy of my birth cert to get a student loan and discovered the "truth" - even my passport was wrong and my parents had to sort out the first few of those and they should have known better! I was born in 1970 and no one noticed that I misspelt my own first name for 20 years.