Deliberate mistake: When Raza is chasing the "empty" train, as it goes over the trestle just before he stops it, you can see the engineer in the train's engine.
Factual error: In the scene where Lee Marvin states that Burt Lancaster loves explosions, Burt Lancaster makes a remark about the big bang theory. He says: the whole universe started with a big explosion. The film is set in 1917 which is 10 years before Georges Lemaître launched the big bang theory.
Continuity mistake: When rescuing Burt Lancaster whilst tied upside down in the canyon, watch the two bandits behind Lee Marvin when he jumps down, as they retrieve the rifles stood up against the rocks twice.
Factual error: Burt Lancaster has a roll of electrical tape on his belt when he is setting the dynamite trap, which wasn't invented until 1946.
Answer: For each member of the Professionals.