Joan exits the taxi during her first scene of "Charlotte" (in black-and-white) and the right rear door remains open. After the cut (and shift to color) the door is closed.
Joan receives legal papers at her Brentwood home, with the address of 246 Bristol Court seen on the cover letter. The actual address of her house is 426 North Bristol Avenue.
When Joan violently throws the vase and flowers at the hospital window, the Venetian blinds appear unaffected. In reality, they'd be severely bent.
The messenger tells Crawford that the information in the envelope is "above my pay grade." The earliest recorded use of that term was in military circles in 1981; it did not begin to enter general usage until the mid-1990s, and finally became widespread when Barack Obama used it in 2009.
There are two references to Joan Crawford checking into Cedars-Sinai Hospital. However, despite their legal affiliation, Cedars of Lebanon hospital and Mt. Sinai Hospital did not merge into one medical facility until 1976. No one alive in the 1960s or early 1970s would have referred to either hospital as "Cedars-Sinai". It would have simply been Cedars of Lebanon or Mt. Sinai.
The General Electric phonograph shown in Bette Davis' motel room before the opening credits is a 1970-ish model and the scene is from the filming of "Hush...Hush, Sweet Charlotte" (1964) somewhere around late 1963 or early 1964.