In the 1935 time line (11 minutes into the film), Ben Golden (Bert Freed) and Marty McDonald (Wesley Addy) are walking past a row of buildings in the studio discussing Baby Jane's acting. There are window air conditioners in almost every upper floor window of the 2-story building behind them. But the first window air conditioner wasn't marketed until 1938, and it wasn't until 1947 that they were mass produced.
Daddy Hudson announces that Baby Jane dolls can be purchased after the show and guarantees them to be exact duplicates of Baby Jane Hudson. The dolls in the foyer do not look like Baby Jane and are not identical to each other.
When Blanche removes the note she has typed to throw to her neighbour from the typewriter, the machine's carriage is at the far right. The next shot is a close up of Blanche adding a handwritten footnote to the letter - the typewriter carriage is now positioned centrally.
After Jane kicks Blanche, Blanche falls and Jane walks around her to kick her again, but when she kicks her again, she is in the same position that she was when she first kicked her, obviously as she kicks her from the right side of the screen (she would be on the left side of the screen if she walked around her).
Mrs. Bates pays her first visit to the Hudson home during early morning hours (Jane is fixing breakfast) but when she returns to her own home next door supposedly just minutes later, outdoor shadows indicate that hours have passed and it's now early-mid afternoon.
When Edwin first visits the Hudson home, the wrought iron gates at the front door swing inward as he enters. But later in the story, when he bursts out of the house after discovering what Jane has done to Blanche, the gates swing outward as he exits.
Both scenes where we see Jane carrying a body in the wheelchair through the kitchen side door (once the maid, then her sister), have obviously been shot at a very short interval : we see two mops on the staircase rail exactly at the same place. In both scenes, on the first shot when she goes down the few steps, the mops are next to each other, then seconds later when seen from inside the car, the mops are separated by a few inches.
When Jane is show crying moments before her "Alright Blanche Hudson! Miss big, fat movie star!" speech, Blanche changes position when ringing the buzzer. A close up shows her hand first extending to the buzzer from the left side of the table. A moment later she's seen sitting in her wheelchair buzzing from the right side of the table. The two shots are a split second apart, so she wouldn't have had time to maneuver all the way around the table in a wheelchair to a new position.
At the conclusion of the film, a newspaper headline about Elvira's death indicates the sisters' house had been in Ventura. However, when Flagg flees the home to alert the authorities about Blanche's terrible condition, he is at the corner of Larchmont and Clinton (as seen by the street signs). This intersection is just south of Hollywood and approximately 60 miles southeast of Ventura.
A graphic after the credits identifies the film's main narrative as beginning "Yesterday", though the events appear to take place over the course of several days and nights. However, it has been debated that "Yesterday" can merely be a synonym for "in the past".
When Blanche admits having been driving the car to kill her sister, she says she broke her spine in the crash. But as we see the scene at beginning of the film, it's hard to believe that she had enough momentum to have her spine broken.
When Blanche is on the telephone after she has made her way down the railing (and right before Jane shuts the door, causing Blanche to realize that she has returned) her right leg moves, obviously voluntarily.
The goof items below may give away important plot points.
In the film's final scene, when Baby Jane walks up to the ice cream vendor, her lipstick is much lighter than in the rest of the film. Moments after, her lips are once again covered with darker lipstick. Additionally, during the dancing scene subsequently on the beach, even though it's obviously hot out, the ice cream shows no signs of melting whatsoever.
The plot hinges on fact that Blanche was supposedly crippled when a car driven by Jane ran into her and penned her to gate. In reality, Blanche was injured while driving a car and unsuccessfully tried to run over Jane, then crawled in front of car after Jane fled. Blanche's injuries would not be consistent with someone who had actually been struck by a moving vehicle as she claimed.
When Blanche sneaks into Jane's bedroom to find food, she consumes a box of large chocolates, yet you can clearly see not one of them enters her mouth and when she picks up a wrapper it is obviously empty and she doesn't even open her lips wide enough to fit a chocolate in.