In some very early episodes, the face of Patty Duke's stand-in can be briefly glimpsed.
In the show's opening credits for seasons 1 and 2, the optical effect when Patty and Cathy (Patty Duke) pretend to mirror each other, Patty's outstretched arm is seen in front of the door frame. This is impossible because she is standing behind the doorway and the arm should therefore be hidden. However, this error is outside the image's safety area and should have been almost hidden in old analogue television sets.
The theme song has the following verses: "Cathy adores a minuet, / the Ballets Russes and Crepes Suzette." The Ballets Russes shut down before Cathy Lane was born.