When Ray and Martha are sitting by the lake - Ray on the ground and Martha in a chair, her arms and legs change position back and forth from the shot facing them both to the shot showing them at an angle.
In the scene on the bus with the dead victim of Martha and Ray, there is a long shot of the woman's face with her eyes somewhat googly and her tongue sticking out, as you hear the bus driver exclaiming her death, etc. Towards the end of the shot, if you watch the woman's face, you can see her tongue move.
When the chlorine and ammonia is mixed in the lab there is an explosion and a lot of white smoke is released. That white smoke would be very toxic chloramine gas that severely irritates the eyes, nose, throat and lungs. Nobody exposed to it would be able to stand around and talk as shown in this film.
A copy of the album "Whipped Cream & Other Delights" by Herb Alpert and the Tijuana Brass is clearly visible in Martha's apartment. The album was released in 1965, while the movie takes place prior to 1951, according to the message at the end.
When Martha and Myrtle have an argument and Myrtle tells Martha she intends to take "Charles" with her to Little Rock, a microphone is visible right next to Myrtle. It doesn't look like a boom mic, but rather a handheld one because you can definitely see someone moves it a little to catch Shirley Stoler's line.