Factual error: In the scene where there is a train passing, the cars are marked "Burlington Northern." There was NO Burlington Northern until 1970, 7 years AFTER the shooting.
Factual error: In the aerial shot where they are driving over the Lake Pontchartrain Bridge two spans are depicted. In 1963 there was only one span. I know this to be true because I lived in New Orleans/Mandeville at the time. Anybody in NOLA could verify this.
Continuity mistake: Guy Bannister brutally pistol-whips Jack Martin (Jack Lemmon) across the face several times as a warning never to discuss his inside knowledge of the assassination. In the next shot of Jack on the floor, he has no blood or any visible abrasions whatsoever on his face.
Factual error: When Lee Bowers' accident scene is shown, a train is passing in the background. Unfortunately, the type of train passing did not come into being until after 1980 - the scene took place in the mid-1960's.
Factual error: About two thirds into the film, in a flashback scene, a military-uniformed man is at an airport in New Zealand. In the background the silhouette of a 737-300 is seen landing. The film is set in the early 1960s, but the 737 did not enter service until 1967, and the -300 series not until the mid 1980s.
Factual error: In one of the scenes where Costner is interviewing the witnesses in Dallas. One old guy says he was standing on the overpass and saw a man running back toward the railroad area behind the Grassy Knoll. There is then a shot, back in time, to the day of the assassination, where the shot of the man running is re-enacted. You can see him running by a Ford Mustang. The first Ford Mustangs were not released until April 1964. The man was running by it on November 22, 1963.
Continuity mistake: During the scene where Garrison and Mr. X are sitting on the bench, Garrison turns his head slowly towards Mr. X's direction. At the next shot he's still, and his head is back to where it was.
Visible crew/equipment: During one of the railroad men's eyewitness accounts of what he heard at the time of the assassination, for only a second on the extreme far right of the screen during the obviously re-enacted footage of the fateful motorcade, you can see just ahead and on the driver's side of the Lincoln limosuine the white sedan lead car; however, you can see it is mounted with what appears to be a boom microphone and other equipment and crew members in the car - one has a headset on.
Continuity mistake: In the scene where Jim Garrison is interviewing David Ferrie in his office, while Ferrie is being escorted out of Jim's office, the camera goes to the T.V. Notice the light in the background behind the T.V. The light in the background is pitch black, and then in an instant, the light comes on as if someone turned on a light.
Continuity mistake: After hearing about the death of JFK in a New Orleans bar, Guy Bannister & Jack Martin walk all the way back to Bannister's office in a pouring rain. As we see the two go inside the office building into Bannister's personal office, their overcoats and hats are completely dry.
Continuity mistake: When Jim Garrison goes to Angola Prison to interview Willy O'Keefe, Willy tells of a time before the assassination of JFK where he was alone with David Ferrie, Lee Harvey Oswald, Clay Shaw, and two Cubans. When we see the flashback, there are a total of six Cubans in the room, not two.
Continuity mistake: When Kevin Costner is talking to Walter Matthau on the plane, in the first shot of Kevin, he has his glasses on; in the very next shot, he doesn't.
Other mistake: We can see a Christmas tree in the background at Kevin Costner's house when the team of attorneys are there going over the photographs, etc. Kevin Costner then meets up with Tommy Lee Jones on Easter Sunday. Christmas and Easter Sunday are not this close together.
Factual error: In the scene where police officers arrest Lee Harvey Oswald in the movie house, the movie that is playing is "Night Of The Living Dead." However, that movie wasn't released until about five years after the Kennedy assassination.
Factual error: The day of the assassination, Guy Bannister, thrilled with Kennedy's death, sits in a bar with Jack toasting "Camelot in smithereens." The Kennedy administration did not come to be known as Camelot until Jackie Kennedy mentioned her husband's fondness for the musical in interviews several weeks after the assassination.
Continuity mistake: At one point during the meeting between Kevin Costner and John Candy, Candy bolts out of his chair and the sunglasses he was wearing fall off. In the next cut Candy is wearing them again without having picked them up. [This scene is edited from a longer scene (that is included on Disc 2 of the JFK "Oliver Stone Collection" DVD). In that sequence, Candy sits down and puts his sunglasses back on. Still a mistake, though, but there's why.]
Factual error: Throughout the movie, it's been said that President John F. Kennedy's motorcade route had been changed at the last minute. This is not true. The route was set in stone for days. In fact, assassin Lee Harvey Oswald supposedly found out that Kennedy was going to ride right past Oswald's work building from a newspaper days before. He rode the same route the newspaper published.