(at around 1h 18 mins) As Munny is getting beaten in the bar below, the Kid and Ned begin to flee the room. Ned falls out the window and rolls off of the wet roof and is heard landing in the mud on his back. When he jumps up, he's completely dry and not a bit of wetness, or mud/dirt on his back and arms.
(at around 41 mins) In the shot following W.W. Beauchamp's leak the planks are dry again with no puddle to be seen.
(at around 1h 55 mins) When William Munny walks in and confronts the posse his hat and clothes appear completely dry, although he has just come in from the pouring rain.
(at around 1h 35 mins) When given back his Spencer rifle by Bill, Ned's grip on the rifle jumps between shots.
(at around 1h 13 mins) At the first time in the saloon, Munny pushes the glass on the table until it touches the bottle. The next shots show the glass a little distant from the bottle.
When English Bob was getting disarmed behind one of the whores you can see the American flag with thirteen stripes and fifty stars this flag was not implemented until July 4th, 1960.
(at around 1h 12 mins) In the film, Ned visits the women upstairs at Greeley's as if it's perfectly normal. While this might have gotten him killed East of the Mississippi River, black cowboys, workers, and ranch hands were a common sight in the West. There is no reason why the prostitutes would have turned away Ned's business unless commanded by their pimp to do so.
(at around 2h 5 mins) Spelling error in the credits - Kelly Zombor is credited as "Boom Opertor", misspelling Operator.
Several times during the film Ned Logan's Spencer is referred to as a RIFLE, when it is obviously a CARBINE.
(at around 39 mins) English Bob says that no assassin would dare hold a gun to a monarch. There were, in fact, six assassination attempts made against Queen Victoria, and they were well known at the time. He certainly would have known about them, though it should be mentioned that he was concerned with successful attempts (looking at a monarch causes the hand "to shake"). However, it should be noted that English Bob was mainly saying this to antagonize people and not necessarily because he truly believed it.
When seen from far away, it's obvious that the town of Big Whisky is tiny, with fewer than a dozen buildings in the middle of nowhere. Yet the town has a sheriff with four or five deputies, most on duty at the same time.
However, in a town ruled by a single, wealthy and powerful man, it is quite likely he would have a heavy detail of hired, heavily armed men at the ready.
When Bill Munny visits his wife's grave before leaving his home to join the Schofield Kid, his wife's grave marker says: "Born 1840, Died 1878, aged 29 years." If she was born in 1840 and died in 1878, depending on what month, she would be 37 or 38 years old.
However, the grave marker actually states that his wife was born in 1849 and died in 1978, making 29 years the appropriate age.
(at around 55 mins) English Bob is in jail and Little Bill is reading from W.W. Beauchamp's novel, but a sheet of script is taped onto the page and clearly visible.
(at around 34 mins) When English Bob is shooting the pheasants from the train, strings attached to the pheasants are clearly visible.
(at around 1h 45 mins) After Will and the Kid shoot the second cowboy and are riding away, the middle cowboy (kneeling) clearly fires shots in a different direction than the other cowboys, even though Will and the Kid are riding off in the same direction.
(at around 1h 26 mins) When William Munny wakes up from his fever after being assaulted by sheriff Little Bill he sits outside the shack and talks to Delilah Fitzgerald. You can see the blue sky behind the house through springs in the wood revealing that it's not the same house/shack as in the first shot and the fact that it's a scene-set.
(at around 1h 9 mins) When Will, Ned, and the Kid first ride up to town and see English Bob in the train. Ned offers the Kid a drink. The kid takes the bottle and clearly places his thumb over the top and drinks then his thumb can clearly be seen moving off the top after pretending to drink.
Despite being only 16 years after the Civil War, no one seems to notice that Ned is Black, and what's more, no one mentions it or describes him in that manner. While there may not have been prejudice to speak of in some mining camps, clear delineations of race and class existed in the towns and cities of the Old West.
(at around 1h 35 mins) Set in 1881. When the townspeople are forming a posse, they are discussing who will pay for expenses, and one of them says that the store won't sell them any more 30-30 shells unless they pay cash. The 30-30 was not introduced as a cartridge until 1895.
(at around 33 mins) Although pheasants were not introduced to the American west from Asia until the 1890s, they are present along the railroad tracks in 1880.
Several of the characters, including Little Bill and William Munny, are seen sometimes wearing shirts that button all the way up the front. This is incorrect for 1880/81, when men's shirts were still of the pullover variety, with or without a collar, and a small buttoned placket at the top.
(at around 1h 45 mins) The whiskey that the kid is drinking from under the tree after the killings is not "Southern Comfort" as noted elsewhere. The name on the label identifies it as "Johnathan Collier Whiskey". However there is a problem with that bottle nonetheless. In the close-up when he takes a drink the bottle's contents info can be seen embossed in the glass around the bottom of the side of the bottle. Bottles were not marked that way in 1881.
(at around 1h 45 mins) Near the end of the movie, Schofield Kid is talking to Bill Munny about how it feels that he just killed a man, Woolvett is drinking a bottle of Southern Comfort. Southern Comfort was not sold in sealed bottles until 1889, the film is set in 1881.
(at around 31 mins) When the train first appears, the whistle blows while the locomotive is in the shot, but there is no steam blowing from the whistle, which is what happens on steam engines whenever the whistle is blown.
(at around 1h 5 mins) When Munny is swearing at his horse in the rain (right after the "I would've killed you" scene in the jail house with Little Bill, English Bob and Beauchamp), Munny mutters "Sorry, horse," he can clearly be seen speaking emphatically after the line, but no sound is heard.
(at around 21 mins) When Skinny comes to talk with Bill while he is building his house, you can see a light being reflected from an unknown source to the house.
English Bob takes his beating from Little Bill, on "Independence day" (July 4). A few days later Little Bill dishes out another beating to Will Munny and then days after that, Munny is recovering in a shack/barn where there is snow outside. Since the fictional town of "Big Whiskey" is most likely located in the East high plains section of Wyoming near Laramie, as opposed to the mountain ranges and Indian reservations in the Western part of Wyoming in the 1880s, the average temperature in July is between 85-95 degrees. Even in the highest mountain plains accumulated snowfall would be nearly unheard of in early to mid July. Also there is much rain in the film, and July is the driest month in Wyoming. However, the film was shot in Alberta, Canada and the snow was unexpected.
On several occasions, English Bob pronounces his erstwhile biographer's name as "Boh'champ". As an Englishman, he would have known that Beauchamp is one the few names that have idiosyncratic pronunciations (such as 'Chumley' for Cholmondley, and 'Urkurt' for Urquhart). He should have pronounced it 'Beecham' (with a silent 'P'), especially as he revelled in illustrating his English superiority over his American colleagues.