Even though the copyright of this movie is 1959, there is clearly at least one part this movie that was filmed in 1964 or later. When the private investigator gets the address in "Manhattan" for the elderly "Bee guy" Eric Zinthrop, he calls "Jerry" and tells him to get right on it. The movie then cuts to a young guy in an office and then without any dialogue, he and another guy drive around in search of Zinthrop. As they do, they pass several 1961-64 Chevrolets. This is especially evident when they pull up to the "Ambulance Entrance" and Jerry gets out of the car. There is a 1964 white Chevrolet Impala (three tail lights per side) parked on the right side of the scene. From the appearance of this portion of the film versus the rest of the movie, this part was evidentally added at a later date.
The trivia item below may give away important plot points.
In one of the final scenes, a bottle of acid is thrown at Susan Cabot, but she isn't acting when she throws her hands up after it strikes her. Someone had filled the 'breakaway' bottle with water, and it was so heavy that when it struck her she said, "I thought my teeth had been knocked through my nose!" The fake smoke used to simulate the acid also choked her; after falling through the window, unable to breathe, she tore some skin off along with her monster makeup, leaving a huge purple mark on her neck.