When helping Charles Xavier and Wolverine free Magneto at the Pentagon, Peter Maximoff is seen wearing a Dark Side Of The Moon t-shirt. Magneto is being freed prior to the X-Men travelling to the Paris Peace Accords that took place in January 1973, but the Pink Floyd album wasn't released until March of that year.
When Magneto levitates RFK Stadium, it seen with a baseball diamond and configuration. In 1973 RFK Stadium was not being used as a baseball venue as the Washington Senators had moved to Texas following the 1971 season.
The 1973 events of the film are implied to take place in January, as this is when the peace accords in the film were actually held. When Quicksilver is first visited in his basement, he is listening to "Hello Hooray" by Alice Cooper, from the album "Billion Dollar Babies". Billion Dollar Babies was not released until the latter days of February 1973.
When they were loading the Sentinel onto the train, there was a shot of a military guard with an M16. His helmet was of a modern PASGT style, not the style used in 1973, which would have been the Vietnam era M1 model.
The film is set in 1973, but the private jet with tail number N540EA is a 1975 Gulfstream II. Actually, it is a Gulfstream IIB, which was not approved by FAA until 1981 and uses the Gulfstream III wing (see the winglets) which was not available until 1979. The original Gulfstream II was introduced in 1966.
When they were touring the Pentagon it was obviously daylight outside. Yet, when the security monitors were taken over they said it was regular television feed. In fact, they were showing Sanford and Son. This can't be since in 1973 this would have been the second year it was on television. And thus, would only be on at night and not syndicated.
The purple seats in the lower ring of the upper deck of RFK Stadium were installed in the early 1990s. In 1973, these seats were light blue. Also, the 400s only go as high at 471, there is no section 483.
When the young Professor X goes back to his room after rejecting Wolverine's request for help, it shows his room littered with alcohol bottles, as well as a picture of Raven (in human form), with an ash tray with a rolled joint in it sitting slightly behind it to the right (but in plain view). The camera then cuts to a close up of Raven's picture, but this time there is nothing behind it, the night stand had been cleared off for the close up.
When Star Trek is playing in the background, the episode playing is "The Naked Time." At first we hear a comment that the chronometer is going backwards, then were hear the discussion regarding the intermix formula being only theoretical. In the episode broadcast these events happened in the opposite order.
When Raven is hit by the bullet after diving out of the special meeting room she is struck in the leg on the inside of her right calf. Later in the French hospital we see the bloody gauze and bandage on the outside of her right calf, however Charles pulled the bullet out of her leg and so there was no exit wound on the outside to require a bandage there or to cause bleeding.
When Quicksilver defeats the wardens during the prison break, he puts on his earphones to indicate, he was listening to "Time in a Bottle"; however he couldn't listen to it, because the music would not play in such a high speed.