The wax statue of "Yvonne" is shown with arms straight down through the film, but toward the end, when there is a shot of the real Yvonne facing the wax statue, the statue has its left arm bent with hand on hip.
When Stephen clears away the condensation from the train window, he looks out and sees Rollo. In the following shots, the window is cloudy, then clear again.
When a fly crawls up into Gogol's cobra pitcher plant, the plant's leaves fold upward to trap the fly. Many insectivorous plants are capable of movement, but pitcher plants are immobile; the plant's leaves were pulled up by unseen wires.
When Yvonne takes the place of the wax figure of her, Peter Lorre at first believes he is seeing the dummy. But Yvonne's dress is different from the one the dummy is wearing.
Throughout the picture, the wax figure moves slightly whenever Frances Drake is subbing for the actual statue. Most noticeable when the bird lands on her shoulder, making the "lifeless" statue sway.
When Rollo goes to the guillotine, the device is shown against the sky. The "sky" has seams in it, revealing it to be a backdrop.
When Gogol disguises himself as Rollo, he does not have an accent when he speaks (whispers). If he did, the audience (and Madame Orlac) would know immediately who he really was.