Factual error: In the scene where the spies are hanging the false backdrop off the bridge, you can see that the railway lines they cross have concrete sleepers, which is out of period (railway lines at this time would have had wooden sleepers).
Factual error: After they pick up Truly Scrumptious in the newly refurbished Chitty Chitty Bang Bang, look at the road surface as they run alongside the GWR train. There is a broken white line running down the centre of the road. These types of markings weren't introduced in England until the blackouts of WW2.
Factual error: There is a scene where Chitty is pacing the train, which is an 'Austerity' tank engine. This engine was made during WW 2 but the movie takes place in the early 1900s.