Continuity mistake: There's an oscillating fan near the ceiling that alternates between running and not running from one cut to another in one scene.
Continuity mistake: When Miss Lonelyheart brings home a younger man, she slaps him on the left side of his face, but when he leaves and stands outside her door, he rubs the right side as if it hurts.
Continuity mistake: James Stewart uses a camera flashgun to defend himself by repeatedly temporarily blinding Thorvald in the closing scene. The number of flashbulbs in the cardboard box drops by two between cuts as Stewart wheels himself back away from Thorwald.
Continuity mistake: After James Stewart's second massage, he buttons once in the middle of his pyjama shirt. In the next shot the top button appears buttoned as well.
Revealing mistake: In the shot of the helicopter hovering above the apartment building, the blue screen technique is needlessly revealed by the eratic and physically impossible movements of the chopper. This could have been avoided if the camera shooting the aircraft had been stationary. It appears, however, that it was a handheld shot.
Continuity mistake: When the nurse first visits Jefferies, his hands are separated. They jump together in the next shot when she places the thermometer in his mouth.
Revealing mistake: At the end of the movie, when Jimmy Stewart is hanging in the window, we can see that it's a stunt double.
Continuity mistake: When a man carrying a big block of ice passes by the sculptress's backyard, we see her cat sitting on a chair next to her as she works on her "Hunger" sculpture. Before the jump-cut, the cat is facing toward the man, and after it, it is facing away him when it gets up, turns around, and meows at him.
Continuity mistake: When Jimmy Stewart is watching the couple in the next building, the wine glass behind him alternates between being almost full and almost empty in successive shots.
Continuity mistake: If you look carefully, you can see that the apartments of Miss Lonelyheart and the Thorvalds appear to be ridiculously shallow in depth.
Continuity mistake: In the scene where Lt. Doyle quickly drinks his brandy, he spills a fair bit on the lapel collar of his pale grey suit. He wipes it off and turns to go, but as we see him leaving, the suit suddenly looks quite dry. It could not dry off like that in only a few seconds.
Continuity mistake: After giving Jeff a massage Stella wipes her hands with a towel. The way it folds changes between shots.
Continuity mistake: When Miss Torso, walks toward her mini stove for the kettle, her hair is in a bun as she walks back to the table and eats and exercises. While James Stewart is on phone with his editor, Miss Torso's hair is suddenly secured in high ponytail hanging down her back as she dances in her apartment.
Continuity mistake: When Stella first comes to Jeff, she helps him back into his wheelchair after a very quick massage. Just before he's helped into the wheelchair his pyjama top is undone, and then when he's actually in the chair, his top is buttoned.
Deliberate mistake: How does Lisa appear resplendent in that perfectly smooth wrinkle-free negligee when we saw it earlier all stuffed into her overnight bag?
Factual error: Jeff has been in a cast (for a broken leg) for seven weeks and has been in his apartment for most of that time. He tells his boss that he has one more week before the cast is removed and can then immediately go back on assignment as a photojournalist. By this time he should have been transferred to a walking cast and be using crutches instead of confined to a wheelchair, but he is still wearing a full leg cast that goes up to and around his waist.
Continuity mistake: When "Miss Lonely Hearts" leaves her apartment, her lipstick is very bright red, but when she enters the bar across the street, her lipstick is now pale red.
Chosen answer: I think her head was in the hat box after he dug it up from the garden where the dog was digging under the marigolds.
You are correct that it was her head.
raywest ★