Continuity mistake: In the end, when the President is visited by the X-Men, he receives a blue binder, which is laid in front of him on the table. When they have left, you see a shot including the President's desk, and you can see that the only thing he has on the table are some sheets of paper, stapled in the upper left corner, opened up. Then the shot changes to a close-up of the President, and then back again, and you see the blue binder in front of him, closed, and the papers have disappeared. (02:01:40)
Continuity mistake: When Viper confronts Shingen, she removes her left glove twice.
Continuity mistake: When Magneto lands the Golden Gate Bridge on the island, as it hits land it is still very light. When the Brotherhood start to walk toward the island, it cuts to a wide shot of them still walking on the bridge, and it's suddenly dark.
Continuity mistake: When Wolverine crashes into the tree knocked over by Sabertooth, notice that the entire front of the truck gets completely folded up and the windshield breaks into a thousand pieces. In the next frame, Wolverine gets thrown through the unbroken windshield and the front of the truck is practically undamaged... (00:18:05)
Continuity mistake: Near the start when Wade and Vanessa are discussing families, in most shots facing her she's got her thumb lying along his jawline, but in most shots facing him it's sticking up by his ear.
Factual error: In the car at the beginning of the movie, Jean uses her telekinetic powers to switch the radio from a station playing "By the time I get to Phoenix" by Glen Campbell to Warren Zevon's "Werewolves of London." It's 1975, and Zevon recorded the song only in 1978 (although the song itself had already been written in 1975, other artists played it in live concerts in the Fall of that year). (00:01:30)
Factual error: Erik arrives in Argentina at a place called Villa Gesell, shown to be a town in middle of mountains. Villa Gesell is a beach town on the Atlantic Ocean coast, with no mountains.
Continuity mistake: In the scene near the end where Ajax has a knife in the side of Deadpool's head and is punching him, later Deadpool pulls the knife out. In a follow up shot there is no hole in the mask where the knife was, but in a later shot the hole is there.
Continuity mistake: When Pierce first begins to interrogate Caliban in the safe house in Mexico, a large portion of Caliban's albino makeup is smudged below his nose, revealing the actor's natural skin in his first close-up of the scene; after switching to a close-up of Pierce, then back to Caliban, the smudge has been fixed.
Revealing mistake: When the old couple who give Wolverine clothes are first driving towards their house, there is an aerial shot of them driving over a bridge. This is supposed to be set in Canada (drive on the right) but it was filmed in New Zealand (drive on the left). They are driving on the right but the arrow on the road is on the left side.
Factual error: As the X-Men land in Cairo, scenes reveal the destruction Magneto is wreaking around the world, where it is simultaneously daylight in New York, Sydney and Cairo - it is possible for it to be daylight in two of these locations at once, but not all three. (01:46:00)
Factual error: When Magneto enters RFK stadium, a groundskeeper is lining a baseball field. RFK stadium did not host baseball in 1973, as the Washington Senators had moved to Texas to become the Texas Rangers after the 1971 season. In 1973, the only team playing in RFK regularly was the Washington Redskins of the NFL. In addition, the Paris Peace Accords were signed in January of 1973; so there wouldn't be green grass inside the stadium, regardless of what sports were being played there.