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.
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.
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.
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.
(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!"
Towards the end of the scene where McKay is learning to ride the bucking horse he is thrown from the horse, lands, removes the straw from his hair and then removes the straw from his hair again when he is helped up.
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".
When Rufus crashes the party and dares the Major to shoot him, he takes three steps down the stairs and is directly in front on the Major. When he turns to leave he takes ten steps just to get back to the stairs. However, since he walks past the major to leave through another door, he actually never returns to the same path on which he entered.
When Buck shoots at McKay in the duel, he causes a wound at least an inch long on the side of his forehead. When Rufus Hannassey is challenging the Major to single combat a little while later, McKay's face is shown full face in one shot and there is no trace of the wound.
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.)
In the barn scene, Alfonso Bedoya puts a blanket on the horse. The horse promptly turns his head and pulls off the blanket. Gregory Peck can be seen suddenly turning away from the camera, and putting his hand over his mouth to suppress a laugh.
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.