Continuity mistake: In a scene where Xavier is pointing a gun at Magneto the distance between the gun and Magneto's head changes between shots.
Other mistake: The knife used in the Argentina pub scene never has any blood on it, despite being driven into one man's stomach and another man's hand-twice.
Visible crew/equipment: In the scene where Sebastian Shaw is revealing the Magneto helmet to Emma Frost in the submarine, you can see the microphone in the helmet's reflection throughout the entire scene.
Factual error: When the US & Soviet navies attack the mutants on the beach they fire a large volley of surface to surface missiles. This is supposed to be 1962, the Cuban Missile Crisis, and ship launched surface to surface missiles had only just been introduced and weren't very common. But what makes this a technical error is that in the barrage we see RGM-84 Harpoon anti-ship missiles, introduced in 1977, and RGM-109 Tomahawk missiles (could be anti-ship or land attack) developed in the 70s and first entering service in 1983.
Other mistake: When Shaw enters the CIA research facility, he grabs a man by the collar and tosses him into the air, yet he never comes back down.
Factual error: A Westland Bell 47G helicopter with the a registration of G-CHOP is seen to land at the Russian army camp/military headquarters when Charles, Magneto and the X-Men are approaching. The registration denotes it is a British aircraft, so unlikely to be in the middle of Russia, and this particular model was built in 1966 - four years after the Cuban missile crisis.
Continuity mistake: In this movie, Emma Frost is seen as an adult woman [roughly around the same age as Professor X and Magneto]. But the character is also in X-Men Origins: Wolverine, set years later when Professor X and Magneto are much older, but in that movie she's younger than she is in this one. Although in Origins she is Kayla Silverfox's sister and her surname is never heard - the character was advertised and created as Emma Frost.
Factual error: When sitting in Xaviers mansion there are cushions on the sofas made from a Sanderson fabric, Dandelion Clocks, this fabric was designed in 2009 but the film is set in 1960.
Factual error: The aquarium in the lounge area of CIA facility where the young mutants gather has several blood parrot cichlids in it. This is a hybrid ornamental fish developed in Taiwan in the mid 1980s and did not exist in 1962.
Factual error: In one scene when Sebastian Shaw talks to Emma Frost in the submarine you can see the TV playing in the background, and right next to it is a Revox A-77 reel to reel machine. This movie is set in 1962, but Revox didn't introduce the A-77 until 1967.
Factual error: There is a pinball machine in the lounge area of the CIA facility where the young mutants gather. The pinball machine's name is "Fun Land", it is a 1 player (1 at a time) machine made by D. Gottlieb and Company in 1968, and did not exist in the film's 1962 timeframe.
Factual error: During the bar scene in Argentina, Magneto uses a dagger he took from one of the former SS Soldiers. It appears to be a Reichsarbeitsdienst dagger with the inscription "Blut und Ehre." This is incorrect as the inscription should read "Arbeit Adelt." "Blut und Ehre" was inscribed on Hitler youth knives as that was their motto.
Continuity mistake: Towards the end, when Moira is pushing Xavier's wheelchair in front of the X-Mansion, when they stop, Xavier's wheel's position change between shots without them moving.
Other mistake: Near the end of the film, Magneto finally kills Shaw by pushing a coin through his brain. The blood on the coin is already dry when it hits the ground, even though only seconds have passed, and since we're shown blood leaking from the entry wound later, we know that it wasn't dry to begin with.
Factual error: Miniskirts weren't in fashion until at least 1965-66, and the high rise hemlines worn by many female characters weren't around until '68 or '69.
Continuity mistake: When Magneto is in Argentina, the two guys sitting at the bar change the distance between each other, between shots.
Continuity mistake: In the original X-Men movie, Professor X wonders why he can't read Magneto's thoughts, only to learn that Magneto has his special helmet. First Class contradicts this by establishing that Xavier knew about the helmet in 1962.
Factual error: The scene at the beginning is set at a German concentration camp in occupied Poland in 1944 (as annotated). When Erik enters Shaw's office, the song "La Vie en Rose" by Edith Piaf is playing. The song wasn't written until 1945 and wasn't recorded until 1947.
Factual error: When Erik pulls the bullet out of Xavier's back, the bullet should be covered in blood, but it is completely dry.
Continuity mistake: When Erik is about to lift the submarine out of the ocean, most of the hand he is using isn't that clean. However, after lifting the submarine out of the ocean part way and the shot cuts back to Eric, most of his hand is clean.
Suggested correction: Xavier has frozen Shaw in place with telepathy. Thus, the reason why the blood appears dry on the coin, is because it is still Shaw's essence, and is still technically frozen in place.
Telepathy doesn't physically freeze a body. He is forcing his mind to stand still.
lionhead