The taxi used by Jim and Christine when leaving the ball has 'Taxi' on a paper sign in the windscreen which is not there in long shot.
When Margaret tells Jim that Christine has gone, and will be taking the train, she throws one end of her scarf over her left shoulder. But in all subsequent shots, her scarf is only seen hanging over her right shoulder.
In the final chase and run to board the train, there would have been no time or opportunity for Jim to write 'Welch Go Home' on the back of Welch's car.
Obvious stunt doubles are used for Bertrand Welch (Terry-Thomas) and Professor Welch (Hugh Griffith) when riding the scooter chasing Jim Dixon (Ian Carmichael)
On the scooter, in close-up, Professor Welch (Hugh Griffith) has his right hand holding his hat, but in the shot of the scooter going through a ford, his arm is by his side.