When the young Amsterdam throws the knife in the hole, it isn't wrapped. When he returns 16 years later, it's wrapped in old clothes.
The scar on Amsterdam's face from Bill the Butcher's sword disappears and reappears throughout the film.
At the hanging, when the condemned man at left of screen is calling to his son and delivering his last words, all four men already have nooses around their necks. After he ends his speech and the hood is pulled down over his face, the nooses are dangling free in front of the other three men, and the hangman places the nooses back on their necks.
When Amsterdam is on the table at the Pagoda and Bill the Butcher throws the cleaver in the air, the cleaver is spinning the wrong way to land and stick in the table, yet it does exactly that.
When Amsterdam and Jenny are kissing after the dance, Jenny says she shouldn't take off her corset because it would take to long to re-lace. In an earlier scene, her corset had a metal closure called a busk in the front, meaning it didn't have to be re-laced every time.
US Navy vessels never fired on New York City during the draft riots.
Barnum's museum didn't burn during the draft riots, it burned in an unrelated fire of unknown origin 2 years later. Both fires happened on July 13th.
When the Union soldiers take aim at the rioters, each man steps forward with his left foot. During the Civil War, on the command "aim" a soldier would step BACK with his RIGHT foot, bringing it behind his left. Stepping forward would desecrate "the line" (all-important in nineteenth-century warfare).
The orders given to the Union soldiers responding to the riots are incorrect for their results. First, the officer in charge orders his column to halt with "Troop, halt!" However, a 'troop' is a cavalry unit, while the one in the scene is an infantry one. Depending on the rank of the officer and the size of his unit, the correct unit designation would be either 'company' or 'battalion'. The soldiers then level their rifles at the rioters on the command 'Present Arms.' 'Present Arms,' however, is a salute; 'Aim' was the proper command for aiming rifles in battle, and would only be given after another series of commands preparing the troops to fire and detailing the selected firing procedure, which is absent in the scene. Also, as the troops approach the mob, they halt before the word 'halt' is spoken. Troops, at least in that era, would march until the word is spoken; they do not stop in anticipation of the command, and this is still practiced in most modern armies as an essential part of discipline.
The cannons on the ship are too small in proportion to both the range and the explosive force of the shells that they fire.
In movie, the Five Points are Anthony, Orange, and Cross Streets. By the time the events in the movie take place, they had been renamed Worth, Baxter, and Park Streets, in an effort to change the area's reputation. Most Five Points residents would have still known and referred to the streets by their original names.
During the Draft Riots, a voice sounds like it's coming through a PA system. Microphones were first developed in the 1870s as part of the telephone, and mics capable of being a loud PA system were not in regular use until the early 1900s. However, the voice begins when a telegraph is on-screen. The words are being sent over the telegraph, not shouted to the crowds during the riots.
When Jenny says to Amsterdam and Johnny "I'll leave you in the grace and favor of the Lord," Amsterdam is on the left and Johnny is on the right. When the camera pans back to them, Johnny is on the left and Amsterdam is on the right. The shot is a pan, not a jump cut. When Jenny says "grace and favor etc." Johnny has moved from Amsterdam's left side to his right side.
Crystal eye balls are fixed and always look to the front without moving. However, The Butcher's eye moves in conjunction with his good eye, revealing that it is in fact a contact lens.
When Amsterdam and Jenny fight before they have sex, Amsterdam slams her against a wooden beam in her room. The black finish comes off the beam several times, revealing the white foam underneath.
When the mob throws rocks at the police, a heavy-looking rock bounces off an officer's head, then bounces around as it hits the ground, as if it weighs nothing at all.
Before the opening battle, Priest and his followers stand in Paradise Square awaiting their adversaries. Bill's men appear from behind some buildings, with a number of large treetops behind them, as if a forested area is behind the square. After the battle, the camera pulls back to show the Five Corners district in the heart of mid-1840s Manhattan. As the shot zooms out and increases in altitude to reveal the computer-generated scenery, no CGI trees correspond with the trees built to represent the Paradise Square area.
Members of The Dead Rabbits dye red stripes on their shirts to show their alliance. The dye clearly ends at the seam that joins the sleeve to the shirt front, meaning that the fabric was dyed red before the shirt was sewn together.
When the competing fire companies arrive at the house fire, one fireman wears modern-day fireman's pants. He may be a real-life firefighter, in the scene as a safety precaution.
In the opening scene, as Priest and Amsterdam ascend through the catacombs, a priest saying mass elevates the chalice while facing the people. Before the 1960s, priests said mass with their backs to the people.
Prior to the street battle in 1846, Priest Vallon recites a portion of the Prayer to St. Michael, "St. Michael the Archangel, defend us in battle! Be our protection against the wickedness and snares of the devil." The prayer was written by Pope Leo XIII in 1888.
Johnny tells Amsterdam that Bendrick the Cockroach "carries a germ" and "if you try to leave the gang, they say, he hacks up blood on you." The link between germs and disease wasn't known until after Robert Koch developed Koch's Postulates in 1876. Louis Pasteur's work, also published in the 1870s, completed the proof to the medical community, though it was called "the germ theory of disease" as late as 1914. In the 1860s, no ragamuffin street kid would have made the connection. The first recorded use of "germ" outside medical literature is John Tyndall's Fragments of Science for Unscientific People, published in 1879. Until then, "germ" was the part of a seed from which the plant sprouts.
Bananas were available in the United States after the Civil War, not during it.
When Bill the Butcher talks to Boss Tweed on the docks as the Irish come off the ships, his words don't match his lip movements.
When Tweed and Bill the Butcher first talk in Tammany Hall, Tweed is feeding his birds as he confronts Bill's abuse of the Irish. The line beginning, "You may or may not know, Bill...", clearly does not match his lip movements.
In the final fight scene, when the camera circles around Vallon in the white smoke, the circular camera tracks are clearly visible at the bottom of the shot.
When Bill the Butcher tells Boss Tweed about the streets that make up the Five Points, he names five. There were three; two continuing through and one ending. Little Water and Mulberry Streets were nearby, but not at the intersection itself.