Factual error: As Martha is lying the table near the beginning, she is singing Danny Boy. The song was published in 1913, long after the time period of the movie.
Other mistake: When the three men meet Harmonica at the railroad station, Woody Strode removes his duster and the shootout occurs. When Harmonica meets Cheyenne and his henchman at the outpost, they are wearing three dusters, each with a bullet hole in them. How could there be a bullet hole in three dusters when Woody Strode removed his before the shootout?
Continuity mistake: Towards the end, right before the shootout, the hole in Harmonica's shoulder is suddenly gone altogether.
Visible crew/equipment: When Fonda and his men leave the train on horseback, after capturing Harmonica guy, the whole ground is covered with automobile tire tracks. (01:29:36)
Other mistake: When Claudia Cardinale first gets off the train station clock has the time as 7:55. When she looks at her watch the time on it is around 10:10.
Factual error: The camera rises up the side and over the roof of the Rail Road Office. The roof of the RR office looked like it was 100 years old. It would have leaked like a sieve all over. That RR office would have been a beautiful new building as it was on the interior scenes at that time.
Other mistake: With Harmonica secured in the train, Frank states he will handle 'the woman' personally. As he rides off with some men while the train pulls away, in the foreground modern vehicle tyre tracks can be seen in the dirt. (01:30:00)
Answer: 1) Jill is a prostitute from New Orleans. She seeks out a new life out West. Love is irrelevant here. 2) She was promised a country living, a family, and wealth. That's why she is looking not only for money or gold but also for the reason her family was killed. 3) So Leone can show her beautiful body. 4) She's saving her life. She's a prostitute and I guess she knows how to fake it. Remember: "There's nothing that can't washed off by a hot bath".