Several times in the movie when we see a close-up of the cops driving in their car, you can see rows of houses behind them out the back window of their car, but in the immediate next wide angle shot they're out in the wooded countryside, with no houses anywhere in sight
(at around 19 mins) Close-up shots of Deputy Ramsey show the red lights on top of his car blinking. Long shots of the car show them off.
After the killer is shot in the left leg by the police on the other side of the moving train, he falls down and there is blood on his jeans from the shot and also on the left side of his hood. I assumed the blood got on his hood because he touched it after grabbing the bullet wound on his leg, but in the next scene, when he jumps into the shallow water, there is no blood on his hood at all.
Eddie LeDoux confesses to the five killings in Texarkana but by this point in the film there had been only four murders.
When Peggy and Roy are leaving the school prom, Roy is carrying Peggy's trombone case through the gymnasium. When they're outside of the school walking to their car he doesn't have the trombone, but when it cuts to them getting into the car he has the trombone in his hand again.
At the beginning of the movie a graphic is put onto the screen to draw attention to the fictional date of the first attack. The graphic reads, "SUNDAY, MARCH 3". Later, as a lead-in to the second crime, the graphic, "SATURDAY, MARCH 24" appears on the screen. The narrator's voice adds, "Now, it is a rainy Saturday night, March 24, 1946, 21 days after the attack on lover's lane." 21 days from March 3 would indeed be March 24, but it also would be, and was, a Sunday, just as March 3 was. The second graphic and the narrator's script were incorrect.
A caption places the Perot Theater on the Arkansas side. It is, in fact, on the Texas side.
Following the second attack (the first attack that resulted in murder) the narrator indicates that stock at all the gun stores was sold out by Monday (day after the attack) and that by Tuesday, May 26, locksmiths were overwhelmed with requests for their work. The attack occurred on March 24, so the date should be March 26, not May 26.
When the Texas Ranger arrives in the train station he goes to buy some cigars. The counter has a sign in front that says "ORANGES" written by a Magic Marker or marker style pen. Sharpies or Magic Markers had only been created a year before and were not in popular use until the early 1950s so this sign would have probably been painted by a sign painter.
On several occasions, the killer is shown using a revolver with a silencer attached. Standard revolvers cannot be silenced. However, gas sealed revolvers can indeed be silenced.
Obvious Day for Night effect when Deputy Ramsey finds the first murdered couple. Trees don't cast shadows on rainy nights.
At the very beginning, during the quick wedding scene, the extra that plays the now husband is obviously waiting for his cue to come out of the church, as he's looking down with a serious expression, while everybody else is at least smiling or doing something to act like it was a wedding.
In the sheriff's office, there are some shots of the bulletin board in the background. On the board is the FBI's Wanted Poster for Frank Morris, following his escape from USP Alcatraz. The Alcatraz escape took place in June of 1962, sixteen years after the events in the film.
LInda Mae Jenkins is shown in the hospital with an IV line and drip chamber made of clear plastic. At the time of this film, set in 1946, that kind of IV line would not be invented for years. It should have been shown as made with latex rubber.
The film is set entirely in 1946 (just like the real killings) aside from ending narration from the present day. However, when the students are attending the Junior/Senior Prom on April 14, 1946, a banner at the prom congratulates the Class of 1946-1947. As the prom is being held in the spring semester of 1946, the graduating class would instead have been the Class of 1945-1946.
Closeup camera crew visible on a flatcar during long shots of the train.
When the cop grabs the suspect "Eddie LuDoux" by his belt and
pulls him over to the police car, the cameraman's shadow is seen on them as they walk across the frame
Obvious fire hose rain coming from the left when Emma Lou Cook boyfriend's car is leaving the restaurant.
Cameraman (and his camera) reflected on the wagon train during the slow motion final chase.
Where the Law men are chasing the suspect and firing under the moving train which is showing in slow motion, you can easily see the cameraman on the flat empty carriage getting his shot.