Olive Tree Genealogy raises the topic of nicknames that aren't derived from given names - how we can't assume "Uncle Charlie's" name was actually Charles.
She asks others to contribute similar occurrences in their family tree.
My grandmother Sissie (Feinstein) Newmark was born "Belle". It's not too difficult to guess that one of her brothers gave her the nickname that stuck with her the rest of her life. Equally obvious in its etymology, her younger brother, Seymour, was known as "Babe." There were actually two Seymour Feinsteins known as Babe, first cousins to one another, both youngest childs.
A related topic are nicknames that are derived from the given name, but in a manner that isn't immediately obvious. I went into detail on this in my post on Jewish Mysticism and Genealogy.
One ancestor was named Zvi Dudelsack. He never immigrated to America, but his children passed his name down as "Harry". Another ancestor was named Zev Perlik, and some of his descendants knew him as "William." You have to be multilingual to follow the etymology, but if the Royal Family were Jewish, it would not be surprising if Prince Harry and Prince William had the Hebrew names of Zvi and Zev.