When Sybil and Richard first ride the subway, a man wearing a hat is sitting next to them. Hours later, when they return on the subway, the same man is sitting next to them again.
While Sybil is talking to Richard near the horse carriage, the same woman wearing a brown top passes by five times.
Moments after Sybil walks out of Central Park pond where she'd been standing, her feet are dry and shoes leave no wet impressions on sidewalk as she leaves.
In the scene where Sybil and Dr. Wilbur are at the piano, Sybil begins playing With a Song in My Heart, however her hands are still on top of the piano when the music begins.
When Sybil goes over to Richard's apartment for the first time, she is wearing a heavy coat of pink lipstick. When she runs away from him a few scenes later and he finds her, she is wearing red lipstick. Then moments later when they are on the subway home, she is wearing dark pink lipstick again.
In present day scenes (set in mid Seventies), Sybil appears to be in anywhere from 25-to-30 years old. But in flashbacks to her childhood scenes when she's around 5 years old (which would be either late 1940's or early-mid 50's), everything (cars, fashions, hair, etc.) appears to be set in 1930's - long before she was even born.
In the scene where Sybil first goes to see Dr. Wilbur and explains what happened in the water, we hear her say "... and I was in the water." But her mouth says something different.
When Sybil (as Vanessa) plays "With a Song in My Heart" to Dr. Wilbur, we hear Sybil play a few notes of the song, when her hands are still on top of the piano.
While Hattie is playing the piano near the end of the movie, her arm movements clearly do not match the rhythm of the music we hear.
Dr. Wilbur repeatedly mispronounces Dr. Quiñones' last name. She says "kwin-NO-ness"; it should actually be "keen-YO-ness".