When the saint gets out of a chauffeur driven car, driven by Freddie Jones you clearly see that the driver is a slimmer dark haired man, while Freddie is stockier and has Grey hair, it then cuts back to a close up of Freddie.
Simon Templar gets out of his car in a London shopping street, as he crosses the road and someone tries to run him over, the street has changed to completely different buildings.
In the final act, when the Rolls Royce arrives to pick up the Saint, he looks to his left from his front door and sees only the front of the car protruding from behind a wall however when he walks to the car, it is parked right outside his home and would have been in full view to him from the doorstep.
When talking to his friend Dinnie in the shooting range Dinnie is using a revolver, which would normally have 6 shots. Dinnie fires 4 and then the saint picks up the gun and fires 3 more.
At around 42 minutes, there are creases/wrinkles visible in the sky at the cemetery.
When Templar is fighting his assailant at the finale you can clearly see a security crew member in a cap in the burning cellar below.