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)
Other mistake: King Arthur is on the far right of the scene as Guinevere is led away by the guards and he nods at someone off camera, asking them if it's OK to start acting as the camera moves around.