During passenger unloading from the stage, the little girl in the pink dress gets off the stage twice.
(at about 15 minutes) When McKay is being turned loose from the Hannassey boys' lariats, one of the "boys" throws McKay's hat on the ground in front of him. The crown of the hat is clearly crushed, as one would expect - considering the abuse it's received at the hands of the Hannasseys. But when McKay picks up his hat at 16:27, it's miraculously in perfect condition - certainly "a better hat than I thought!"
When Steve Leech first picks up Jim McKay in the buckboard he drives past Buck Hannassey and gang standing outside the saloon mocking Steve. The sun is nearly down as the shadows are very long. Steve then drives around the corner to Julie's house and now the sun is directly overhead.
When Steve Leech (Charlton Heston) starts out to catch the major, the sky is deep blue and clear, but when he catches him just seconds later, it's a cloudy sky. Steve's bandana is also tied differently. Clearly the scene was shot in more than one take.
There are two kinds of mouth pieces cowboys use to handle a horse. In the scene where James McKay keeps on mounting and falling from that horse Thunder, sometimes the horse is using a "bridle" and sometimes he's using a "brake".
McKay tells the Major that the "hazing" he received from the Hannasseys upon his arrival was to be expected, stating that he was "keel hauled" as a young seaman the first time he crossed the equator. This is highly unlikely, since keel-hauling was a form of punishment which often resulted in death, never a form of hazing. It was so severe that it was abolished in 1853, at least 25 years before the time this film is set (based on the Colt revolvers that the cowboys are carrying). Keelhauling is depicted in the film Mutiny on the Bounty (1962).
The Major says of the dueling pistols McKay gives him as a gift, "Made by John Nock of London--none better". "Henry" Nock was the famous London gunsmith who was renowned for his fine weapons, both dueling pistols and shotguns, not "John".
The film's opening credits further the typical Hollywood misconception about stagecoaches. Stagecoaches never operated at top speeds as shown, with the wheel bearings smoking from the generated heat. The team of horses would have dropped dead within a few miles. Stagecoaches operated at a slow trot almost all the time - about 4 or 5 miles an hour. The horses still were typically changed every 10 to 15 miles.
What made travel by stagecoach so fast, relative to anything else, was the relentlessness. The stage stopped only long enough to change horses and didn't need to wait for the worn team to recover. Only a railroad could beat them for distance over time for a passenger. The only time they achieved the speeds shown in the opening credits were when they ran down a hill to gain speed to go up the next hill - and that was considered very dangerous.
In the opening scene, two horses are running at the front row in the wide shot. In closeup shots, more than two horses are seen to be running at the front row.
The knife flies toward the log before Buck brings his arm forward to throw it.
The S-shaped portion of the canyon where Hannassey's men trap Terrill and his ranch hands between barriers is the same location (supposedly further down the canyon) where Terrill later walks to confront Hannassey. A large round, white boulder can be seen half-way up the middle of the far canyon wall in all of the aerial shots and over Gregory Peck's head in the final ground-level shot when the shoot-out concludes.
At the very end of the movie, at around 2h 45 mins 45 secs, when McKay and Maragon contemplate the landscape upon leaving the canyon, a car (it's too fast to be a horse) can be seen driving in the background right in the middle of the image and the two protagonists.
At the beginning of the film Peck's character James McKay is mocked for his wearing of a bowler hat, which the characters make out as something only an easterner would wear. This is in stark contrast with history where the bowler hat was one of the most popular styles in the old west, beating out the Stetson and the sombrero. It has even been referred to as "The Hat that Won the West".
In the scene where Gregory Peck first visits Jean Simmons on the "Big Muddy" property and they gaze across the river, a long string of tall high voltage electrical towers can be seen arrayed against the most distant (California) hilltops. They are seen again in a later scene shot from a similar angle. The plot of "The Big Country" takes place in the 1800's.
Some of the men have belt loops on their trousers but belt loops were not invented until 1922.
When Buck and McKay fire the cap and ball pistol, the sound effect is that of a modern cartridge.
When Jim McKay is riding through Blanco canyon to save Julie there is a backhoe tractor visible at the bottom of the screen inside an inlet as he passes.by.
When McKay first arrives in town and meets Julie at the gate, some object (boom mic?) is visible above their heads (center screen). (This is noticeable in the mid-90s laser disc release and was corrected for the 2001 DVD so the boom doesn't show.)
Just before the duel between McKay and Buck Hannassy, Leech says to the Major that they haven't heard a single shot. Yet Rufus Hannassy shot at Buck to prevent him shooting an unarmed man not five minutes earlier.