A few weeks before filming started, Balthazar Getty fell from a tree and broke both his wrists. The director decided to write his injuries into the script, and put his character Ralph's arm in a sling for half of the film.
Sam and Eric (the twins) are never identified by their names throughout the film. Even the official cast list enters them as the Twins. This points out their interchangeability since even in the book (and the original 1963 film) no one could tell them apart.