When Mannix and Johnny are dismantling the entrance to the mine Mannix removes the same board several times.
During the final shootout the Mannix Imposter fires more than six shots from his six-shot revolver.
When the fake Mannix arrives at the airport in the UK all the vehicles in the background are of US manufacture including a Ford Galaxie. There is also a significant smog layer in the background which wouldn't be present in the UK.
When Mannix arrives in London, a car has been arranged to pick him up, and as it should, it has right-hand drive and the front passenger seat on the left. In the background, however, many of the other travelers from his plane are boarding what is supposed to be a tourist bus. They are entering a door on the right front of the bus while the driver is seated on the left - a configuration that would exist in the United States, not in Britain.
Once the fake Mannix arrives at the ghost town there is no longer a reason for him to pretend to be Mannix and his make up is no long necessary.