When Slade is unrolling Kitty's invite/ handbill the first time, the rear is blank. When he reads it the second time, the content of handbill can be seen through the paper.
A leading performer like Kitty would have a dresser to assist her in donning and doffing her costumes. She would not be in her dressing room by herself.
The hand of the first victim moves.
In the final scene, when Slade's body floats by, some air bubbles surface giving the impression that Slade farts.
The police inspector says that a fingerprint was taken from one of the Ripper murder scenes, and the inspector himself carries a vial of fingerprinting powder. However, the Ripper murders took place in 1888; the first criminal identification from fingerprints took place in Argentina in 1892, and the British police did not adopt fingerprinting until 1901.
The police use flashlights which would not be invented for another ten years.
George Sanders says that handguns are illegal in England, "even for the police." In the final scenes he is chasing Creger with, and firing, a handgun.
In the scene at the Black Museum, Inspector Warwick mentions the "four murders" but by this point in the film there have been five.