Visible crew/equipment: After Lucifer talks to God and doesn't know what is happening and Saint Peter comes to escort him out they get into a discussion why he failed the entrance examination, you can see a reflection in glass between the camera and Lucifer and a man who does not appear to be St. Peter. (Maybe director Stanley Donen?) (01:39:25)
Other mistake: After Stanley becomes the nun he demands one more wish, but George informs him that the ice lolly he got as a trial wish counted - which had cost him sixpence, and which George then pays him back. However, on the Post Office Tower, after Stanley saying he'd like to see what Margaret is doing, George lends him a telescope (also costing Stanley sixpence) which is then forgotten about when George reminds Stanley about the ice lolly. (01:30:00)
Answer: It's a joke because the souls already belong to God. It's mentioned during the jail scene. So the question is irrelevant, as God already has well over 100 billion souls in any case, and the devil can't gain them back.
There is no jail scene in this movie. You're probably thinking of the remake.