The following is my email to him and Don Knuth's notes and his validation of the mistake, on the email copy that he sent back together with the check. Bernstein Polynomials are the foundation of the more familiar Bézier curves.
(The Imagen mentioned in the letter was a startup company whose founders were researchers at Stanford. Don Knuth was at one time on the Board of Directors of Imagen. The John Hobby mentioned in the letter was one of Knuth's Ph.D students. )