Mark Evans

Mark Evans is a character mentioned in Harry Potter and the Order of the Phoenix.

Here is his one time mention in the novel: "I know you [beat up] Mark Evans the other day" -Harry, to Dudley.

This would not have any importance whatsoever if not for the fact that Lily Potter's maiden name is Evans. Because of this, many people thought that Mark Evans was an important character who would show up in later books. However, J.K. Rowling admitted on her website that Mark Evans was a throaway name she thought of off the top of her head.

Source
J.K. Rowling Explanation