Revealing mistake: Whenever Dr. Nemur is smoking, you can tell the cigarette's not lit, because you can't see any smoke. When Mrs. Kinnian smokes in the movie, plenty of smoke is visible.
Charly (1968)
1 revealing mistake - chronological order
Directed by: Ralph Nelson
Starring: Cliff Robertson, Claire Bloom, Lilia Skala, Leon Janney
Revealing mistake: Whenever Dr. Nemur is smoking, you can tell the cigarette's not lit, because you can't see any smoke. When Mrs. Kinnian smokes in the movie, plenty of smoke is visible.
Charly Gordon: Marry me, pretty girl, marry me.
Alice Kinnian: We will marry at... quarter past Wednesday on the 74th of November, and our anniversary will happily be... on those days where we both remember.
Trivia: This movie is based on the book "Flowers for Algernon." The Simpsons also spoofed this in an episode called "HOMR." (The one where they find that Homer has a crayon up his nose.).