When Dr. Jeckyll comes after Shemp and Bea Bopper with the hypodermic, he isn't wearing his glasses. But in the 3-D effect shot of the needle, he is. In the next shot, the glasses are gone again.
When Norma Randall is first tied up on the operating table, her gag goes between her teeth. But in all subsequent shots, her gag is over her mouth and not between her teeth!
The word "sleuth" is misspelled as "slueth" on the detectives' door, although there are no (other) comic misspellings. It appears to be a genuinely accidental misprint.
The pen Moe throws at Shemp after he does his "B-bopper" routine is supposed to be one of the 3-D effects, but the pen clearly doesn't fly straight when it's thrown at the camera.
A meat cleaver is thrown at Larry. Instead of saying "A cleaver tried to cleave me in half.", Larry says "A cleavage tried to cleave me in half."