Immediately after Hyde changes to Jekyll in front of Dr. Lanyon, he moves his head and briefly reveals the padded armature attached to the back of his chair, intended to hold his head in the same position while the makeup artists worked on various stages of his transformation.
After Muriel's father consents for Jekyll & Muriel to be married the next day, Jekyll goes home and plays part of Bach's "Toccata & Fugue in D minor" on the organ. There is a mid-shot with March playing the keyboard, then there is a close-up of the hands on the keyboard. The close-up hands are an obvious double, as they are playing the piece correctly. March's mid-shot has his left hand ascending on the keyboard while the notes of the music playing are descending.
During Hyde's first visit to the Variety Music Hall, he reaches over the railing to trip a waiter with his cane. As he lurches around to grab his cane, he knocks his top hat off the railing and it lands on the floor next to the waiter. In the next shot, Hyde is holding onto the hat as he lashes out with his cane.
When Dr. Jekyll is making his first portion of the mixture to transform from Jekyll to Hyde the glass is very dirty and you can't see through the glass, but when he lifts the glass in front of the mirror it's perfectly clean.
When Hyde first meets Ivy, he tells her he knows she lives in some pig sty on Baptin's Court. But later, a news item about Ivy identifies her address as Diadem Court. (The closed-captioning in the first scene actually reads "Diadem Court" despite what Hyde is heard saying.)
When Dr. Jekyll is first seen in the mirror, looking directly into the camera, the effect was achieved by filming Fredric March through a hole in the wall framed to look like a mirror. But moments earlier, the trick is revealed when the butler walks past the "mirror" but has no reflection.
When Hyde looks into the mirror through the subjective camera, his "reflection" has two stains on his shirt on what would be the right hand side of the shirt if a true mirror image. However in the next shot where we see both of the Hydes, the mirror image and the real image, the stains are now on the left hand side of the shirt. This confirms that the subjective camera view was achieved by the 'mirror' really being a hole in the wall with the actor facing the camera.
The goof item below may give away important plot points.
After Jekyll turns into Hyde while sitting on the park bench, he repeats the word "death." When he laughs after saying the word, "death" you can see two rows of bottom teeth. The bottom row of March's actual teeth are visible along with the fake set in front of them.
As Hyde makes his escape after strangling Ivy, he jumps off a
stairway landing to the one below. This action is done over two shots. At the end of the first shot, the sound of him landing on an unseen platform can be heard.