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: 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.
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: 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.]