In the buildup to the climax, Shane rides up to Grafton's store, ties up his horse and steps up to the swing doors. We then see little Joey running toward the store and Shane has disappeared. It could be that he has already gone inside, except that in the next shot he is once again standing at the door.
Shane clearly misses the third man in the showdown - he fires level instead of up at the balcony, but the next shot shows him almost standing and firing in the right direction. In the following shot he's crouching again.
At the bar, when Chris Calloway throws whiskey on Shane, there are four different stains on his shirt from shot to shot.
Early on at the saloon where Shane gets the drink spilled on him, his shirt is wet/dry/wet between shots.
During the fight between Shane and Calloway in the saloon, successive shots of Calloway's face show his left cheek to be bloody, then almost free of blood, bloody again, and even completely free of blood in several subsequent shots - all while the fight is in progress.
When Joey is calling after the departing Shane at the very end, his voice echoes. There is nothing around which could possibly produce such an echo.
At the Independence Day celebration, they are flying a US flag with 40 stars. As the stars represent the states, only from November 2 to November 11, 1889 (after South Dakota and before Montana) would this be correct. It is highly unlikely that any of these farmers would have gotten a flag from that narrow time period. It would not have been the correct flag in July.
At the beginning, when Shane rides a horse down the hill, Teton Pass Highway is visible in the background.
After Shane gets in the barn carrying the saddle to set it by, a motorized vehicle drives left to right far on the outside. It appears just above Joey's head.
There is a bus (some say truck, some say car, some say motorbike) moving left to right in the far background in one of the opening shots. (The vehicle appears to have been digitally removed from the current DVD version, although it is still visible in the accompanying re-release trailer.)
At Grafton's Supply Store, Joe Starrett is flipping through "Sears and Roebuck" "Catalogue 112". Catalogue 112 is dated 1905; some 20 years after Shane takes place.
Marian's wavy blonde bob (actually a wig) would have been just what you might expect from an attractive younger woman on Sunset Blvd. in 1952, but is about as inappropriate as you can get for a middle-aged pioneer woman in the wilderness with no nearby beauty salon to maintain the color, the length, and the curl.
When Stonewall leaves Ernie's place after the latter has decided to leave, we see Stonewall riding away. Before the end of the shot, when Stonewall is about 50 yards away from Ernie's place, the soundtrack continues with Stonewall wishing him good luck, sounding as if he were still right in front of Ernie.
The name Starrett is variously pronounced "Stark" or "Start" or with two syllables with the first pronounced like the word "stare".
At the end of the big gun fight when Shane and Joey are talking outside of Grafton's bar, Shane calls him Jimmy.
When Shane is teaching Joey to shoot he says that the handle of the gun should be placed between the wrist (pointing to Joey's elbow) and the shoulder (pointing to Joey's wrist).