Factual error: It is barely credible that a young Victorian woman like Elena would even think about wearing a goatskin miniskirt - exposing her legs in those days would be akin to walking about topless nowadays. Even if she did those bright yellow cotton knickers - gleefully visible in the scene in the beehive - are in no way from the 1860s. Her pants are a hundred years ahead of their time.
Continuity mistake: The police that come to the aid of the professor are driving an older Ford when they approach than when they arrive to help. The car is a newer model in the later scene.
Other mistake: When the parent monster is just about to enter Piccadilly square, for a brief moment a view of a bus is shown. The writing on the bus is reversed.
Audio problem: As Tom and Mary exit a home that faces the town square, the sound of applause is heard even though almost all the townspeople are holding glasses of lemonade and nobody is clapping.
Character mistake: The registration number of the stolen van is GPW729 but every time it mentioned in the dialogue it is stated as GPW927.
Revealing mistake: The Son of Hercules brings to the nearby village the old man that died on the beach. You can see the 'corpse' move his arm to assist Mark Forest as he dismounts him from the horse.