The scenes at the Miami hotel show the sun setting over the water. Miami is on the Atlantic coast, so the sun should set over land. The scene was shot at St. Petersburg beach, on the Gulf coast.
In 1968, Noodles returns to the train station and opens a briefcase full of money. As he gets set to close the briefcase, we hear the chug-chug of a steam locomotive starting up in the background, then a steam whistle sounds. New York City banned steam locomotives in 1860. In the US, most steam engines were replaced with diesel locomotives by 1958.
In the iconic shot of the Brooklyn Bridge between the two buildings, a modern streetlight is visible by the base of the bridge.
When Deborah removes her theatrical make-up, her hands get covered a mix of greasepaint and make-up remover. When she says "I thought you might know why"- "Me? Why me?" they're clean.
In the very first scene, when the woman is shot, her shirt is covered in blood. When she is shot again, even more blood stains her shirt, but when she falls back, her shirt is completely free of blood.
In November, 1968, when Noodles visits the locker in the station and pulls out a wad of US Currency, the top bill is signed by W. Michael Blumenthal, President Carter's Secretary of the Treasury from January 23, 1977, to August 4, 1979.
Just before the boys demonstrate the salt trick to the Italians, fluorescent lighting is visible inside the large buildings in the background. Flourescent lights weren't commercially available until the 1940's.
In the scene at Miami Beach which takes place in 1933, beach goers are seen playing with beach balls. The beach ball was not invented until 1938.
When Noodle stabs Bugsy they are on the left side of the Manhattan bridge. But when the police are coming, the whole scene is moved to the right side of the bridge.
The boys first put money in a locker at the Lackawana Terminal in Hoboken, NJ. When they come out into the street, they are at the base of the Manhattan Bridge in Dumbo, Brooklyn, some distance away.
When the cargo floats to the surface in the scene in the harbor, the crates are floating with the balloons out and on top of the water, offering no extra buoyancy, and therefore not needed.
During the Famous scene where the young boys are crossing the street with the Manhattan bridge in the background, the building on the right has a visible window air conditioner, something that was not available until 1932.
Container cargo cranes are visible when the boys are waiting to for the crates of booze to float to the surface. Containerized cargo was not used until the middle 1950s.
Chief Aiello wears Captains insignia, not police chiefs rank.
A 1950s New York housing project is visible when Bugsy shoots Dominic in the early 1920s.
A guy in 1933 cuts cheese with a knife model Buck 110 that was created in 1962.
During the scene when the young boys set the newspaper stand on fire, they run around the corner and peek through a window in the concrete which has visible spray painted graffiti on the wall, spray paint was not invented until 1949, this scene takes place in the 1920s.
When Deborah's train leaves for Hollywood, a French railroad car (SNCF) is on a track in the background. Above the French car, a sign can be seen saying "Voie 13", which is French for Track 13, indicating this scene was shot at a French railway station.
When Fat Moe gives Noodles the locker key, he opens the door of the clock. The key for winding up the clock is in place. After a quick cut to Noodles, the key is gone.
When young Noodles holds young Peggy in the toilet and she says "You better stop squeezing me, or I'm gonna poop in my pants", the beginning of her line sounds twice, as if two different takes have been overlapped with a cross fade.
When Noodles talks to Deborah in the dressing room after her play, every time she raises her voice it reverberates like she'd be talking in a big hall - which is because the small dressing room is a set, presumably with one wall and the ceiling missing, built on a big sound stage.
The Plymouth Valiant Duster was not available until 1970.
When Max and Noodles are on the beach, Noodles' girlfriend lays a newspaper across his face. When the wind blows, you can see duplicate pages, revealing that it's a fake newspaper.
One of the cars parked outside Secretary Bailey's (Max's) mansion in 1968 was a silver 1971 Cadillac Eldorado convertible.
In the credits, the song Yesterday is "Used by permission Norther Songs Ltd." where it should say "Northern Songs Ltd." (including the second 'n' in Northern).
When Deborah is a child her eyes are brown; when she is an adult they are blue.
Darlanne Fluegel's last name is misspelled in the opening credits.
While Dominic looks at himself in the mirror, just after the boys talk about the deadbeat newsstand vendor, a lady can be seen behind him picking up a 50-star U.S. flag. But the flag in 1920 had 48 stars.
In 1968, Noodles rents a 1962 Pontiac, which is far too old to be in a rental fleet.
Noodles watches a 1967 telecast concerning the investigation revolving around Chairman Bailey (on a decidedly European-looking television set). Twice during the telecast, we see a cameraman with a portable video camera and an assistant with a portable videotape recorder. The very first Electronic News Gathering (ENG) equipment wasn't available in the US until at least 1971.
Early, when the guy is being beaten, the thugs shoot the punching bag next to his head to scare him - the shot is fired with a semiautomatic pistol with a silencer. The thug then puts the silencer in the guy's mouth and cocks the hammer of the pistol - since it is a semiautomatic, the hammer should have already been cocked.
When Noodles is visiting Deborah, during the various cuts when she is telling him to take the back door out, the makeup is almost gone. When she is standing by the door the make up is back in places. Also through the conversation the make up disappears and shows up in places.
When we see Max walk toward the garbage truck and supposedly get into the back to be ground up and killed, you can see the actor's feet running alongside the truck (by the rear wheel) as it starts to move.
When Noodles first meets the adult Deborah (1.45:30) his hair is dry and neatly combed but moments later he walks through a door to first meet Frankie Manoldi and his hair is noticeably oily and more tousled.
When the boys were about to role the drunk I noticed burned out building in the rear not haven't taken place until the 1960's and 70's.