After the operation, when Charly loses his temper over being beaten once again by the mouse, no one seems to notice that he is now pronouncing Algernon's name with the first N included, instead of his previous "Algeron" with the missing N.
The reason that Cliff Robertson was not at the Oscars to receive his Best Actor award was because he was in the middle of filming the movie Too Late the Hero (1970) in the Philippines. The director Robert Aldrich would not allow him to attend the ceremony, as a flight from the Philippines to Los Angeles and back would be too time-consuming due to budgetary restraints. Robertson pleaded with Aldrich, even offering to pay out of pocket for any costs associated with his absence, but to no avail. After Robertson won the Oscar, the crew of Too Late the Hero presented him with a mock statuette made out of wood. According to Turner Classic Movies host Robert Osborne, after the Philippine location shooting was over, Academy of Motion Pictures Arts & Sciences president Gregory Peck greeted the cast as they disembarked at Los Angeles International Airport. Robertson was holding his fake Oscar when he got off the plane. As he was approached by Peck with the real statuette, Robertson threw the wooden "Oscar" over his shoulder. The fake statuette hit Michael Caine in the forehead, causing him to bleed profusely.
Cliff Robertson saw more than one TV production he'd starred in turned into hit movies with other actors (such as Days of Wine and Roses (1962)), so when he starred in the 1961 The United States Steel Hour (1953) production of "The Two Worlds of Charlie Gordon" (later adapted into the novel "Flowers for Algernon" by Daniel Keyes), he bought the rights and later was responsible for turning that story into this film.
The film is based on the 1966 novel "Flowers for Algernon," by Daniel Keyes. The novel, in turn, was based on a short story of the same name written in 1958 by Keyes and published in "The Magazine of Fantasy & Science Fiction" in 1959.
Before Charly improves his intelligence, he writes left-handed, but when he becomes brilliant, he writes right-handed.