Factual error: Prior to WW1, Colonel Ludlow picks up Samuel and Susannah at the train station in an all steel Model T Ford. This style of car did not come out until 1917. The correct car would have brass lights and radiator.
Continuity mistake: When the barkeeper refuses to serve One Stab beer he flaunts his club and puts it on the counter, with the handle towards him. When Tristan picks it up to hit him the club is turned by 180 degrees. (01:04:05)
Continuity mistake: When Alfred confronts Tristan on the porch, the morning after Tristan has done the deed with Susannah, the buttoning of Alfred's shirt becomes reversed, then changes back again.
Continuity mistake: The headstone on Samuel's "heart grave" had writing in the cross bar ("We will not see him, but we shall join him"), as well as his name below followed by "born" and "died" dates. The next three times the grave marker was shown (from a distance), only "Samuel Ludlow" could be seen etched on it. (00:49:47 - 00:54:54)
Continuity mistake: When Samuel is dying and Tristan is sitting over him, from one angle Tristan's hand is covering his mouth, but in the reverse angle it's down.
Plot hole: During the war sequence, when Samuel dies, how necessary was it for the two Germans to go through the trouble of setting up a machine gun to shoot him? I find it hard to believe they had no other weapon but that. It seems to have been done only to build suspense. (00:42:45)
Continuity mistake: When Samuel dies in the movie, if you watch closely there is blood running down his face in only every other shot. (00:43:37)
Revealing mistake: On her first evening at the ranch Susannah watches Tristan breaking in a mare. When he falls on the ground you can see that he merely climbs off instead of being thrown. (00:18:00)
Continuity mistake: When Tristan pushes one of his bootlegger opponents into a pitchfork the spikes are coming out of his chest. The way they were wrestling chest to chest Tristan should have been seriously injured too. (01:51:00)
Continuity mistake: When Suzanna comes in for dinner the first night there, all the guys raise something to give her, but Tristan goes to raise his plate. But cutting back from her reaction, he has only lifted his glass to drink. (00:15:20)
Visible crew/equipment: When Tristan pulls the third German cavalryman from his horse you see the face of Brad Pitt's double who, with his mustache, bears a strange resemblance with Adolf Hitler, something that can not at all be said about Brad Pitt. (00:39:45)
Factual error: The three Ludlow sons were leaving for England on October 14, 1914. When they were walking outside guiding their horses, the small trees in front of the house had green leaves. In Montana, Fall foliage typically begins by early - or mid-September. In most of the state, there should be peak Fall foliage (yellow/orange/red leaves) by early - to mid-October, not green leaves on the trees. Also, some time later when the sons were riding their horses through the field, the leaves were still green. (00:28:21 - 00:32:00)
Continuity mistake: Susannah's hair problems during her first encounter with Tristan after his many years of absence have been stressed already. But Tristan's hair has an even stronger mind of its own, as there is a wide strand combed back covering his parting, which looks as if it has been fixed elaborately, but which nevertheless comes and goes during the scene. (01:25:50)
Continuity mistake: After Tristan's second return he comes to Susannah's house. They make eye contact while he is still standing behind the wall. The camera cuts to Susannah and then back to him, and he is standing in the garden, with no time in between to walk further down to the gate and enter. (01:24:55)
Continuity mistake: Tristan is seen sailing on the ocean. In all panorama shots the boat is a two-master, but when you see him behind the steering wheel there is one mast only. (01:09:20)
Visible crew/equipment: When the British troops try to overrun the German lines soldiers are catapulted into the air by German grenades. At least once you get a good view of something like a springboard that was used for this purpose, that is when you hear Samuel's voice saying :"...the souls of those wounded so terribly...". (00:33:50)
Factual error: It has already been mentioned that, although the war scenes are set in early 1915, gas was not used until April of that year. There are further inaccuracies regarding this particular scene. The mask worn by the German cavalryman is not the sort of mask that was in use in 1915. It looks like a late WW1 or WW2 gas mask. Furthermore, since horses can also suffer from gas, a cavalry horse taking part in an attack in which gas was used would also be equipped with a gas mask. However, by early 1915, when both sides were dug into trenches, cavalry would not have been used anyway.
Continuity mistake: During Samuel's death where Tristan is cradling him in his arms, there is a stream of blood at the corner of Samuel's mouth. The camera cuts to Tristan's reaction, but when it cuts back to Samuel, the blood is gone. In later shots of Tristan preparing to remove his heart, the blood stream reappears. (00:44:00 - 00:44:55)
Continuity mistake: When Susannah and Alfred talk at Samuel's grave the way Susannah's hair is blown back, forward, or not moving at all changes from shot to shot. (00:48:30)
Continuity mistake: When Tristan attacks the policeman who has shot his wife he gets hit on his cheek with a rubber truncheon. However, in the next shot the blood is running from his temple. (01:39:15)
Answer: Cutting locks of hair is often done in memory of the deceased. Knowing of her impending death, she cut two locks so as not to have them tainted by blood, but dropped one on the floor. I imagine it was the one for Alfred, but it is just my opinion.
I have goosebumps with this interpretation. I always thought that it was related someone with "scalping." She kills herself. She is her own victim. I don't know.