The Cunninghams, the focal family of the popular 70's sitcom Happy Days, consisted of father Howard, mother Marion, and three children: Joanie, Richie, and Chuck.
Garry Marshall created the first draft of this show in 1971, and called it New Family in Town. He felt nostalgic for his own youth in the 1950's, and he believed that a nation wearied by the changing times and the war in Vietnam would enjoy the return to a supposedly simpler time. The plot involved teenager Richie Cunningham (Ron Howard), his girlfriend Arlene (Tanis Montgomery), and events surrounding the Cunningham family's purchase of their first television set. Ric Carrott played Richie's older brother, Chuck.
The network executives rejected the show. The pilot episode of New Family ran in 1972 as an episode of Love, American Style, the popular 1970's anthology series, under the title "Love and the Happy Days." It would likely have been forgotten had George Lucas not created the next summer movie mega-hit with American Graffiti, a nostalgia film set in 1962 and featuring Ron Howard. Suddenly, every television network wanted a 1950's nostalgia series, especially one with American Graffiti's All-American Boy star.
After some retooling, Happy Days premiered in January, 1974. Gavan O'Herlihy and Randy Roberts both played Chuck. I find it amusing that a character nobody remembers was played by three different actors.
After two seasons as a rather low-key, somewhat realistic family comedy, Marshall significantly revised the show into a wisecrack-a-minute sitcom with a live studio audience and increasingly fewer references to the actual events and culture of the 1950s. Fonzie, the soft-spoken and slightly scary hood with a heart of gold became a cool, if somewhat preachy loudmouth, and he moved in with the Cunningham family. And Chuck Cunningham, apparently ceased to exist.
No one on the show ever mentioned Chuck again. Even much of the show's youthful audience genuinely forgot him, as surely as kids in the 1990's failed to remember that Geri Haliwell had called herself Sexy Spice before becoming Ginger Spice.
Chuck is a famous example of a retcon in television. In the final season of Happy Days the Cunningham parents mention their two children, Richie and Joanie. Chuck Cunningham retroactively never existed. The unexplained disappearance of a character from a series is now sometimes called Chuck Cunningham Syndrome.
Notes: Scenes from the New Family... pilot appear as flashback sequences in a first-season episode of Happy Days.