(at around 30 mins) When The Devil talks about The Garden of Eden being a swamp like Croydon, and then points saying it's over there, he is actually pointing roughly east towards Islington, whilst Croydon is south from their position on the tower. He could have been referring to Hackney Marshes which are in the direction he was pointing, but they were only marshes in name at the time the film was made.
(at around 55 mins) During Dudley Moore's song "Love Me" which he sings in character as Stanley Moon, the woman to the right of the screen seems to say repeatedly "Oh Dudley" instead of calling him by his character's name, "Stanley".
The goof item below may give away important plot points.
Crew or equipment visible
(at around 1h 40 mins) After George talks to God and doesn't know what is happening and Saint Peter comes to escort him out, as they get into a discussion why he failed the entrance examination you can see a reflection in the glass between the camera and Lucifer.