Continuity mistake: In the first shot of the dead man hanging form the tree, his head appears to be about 12" from the tree limb. After Mattie climbs up to cut him down, his head is several feet from the tree limb.
Continuity mistake: When Mattie crossed the river on horseback, she was submerged to her chin. When she exits the water on her horse, her clothes, gear, boots and hair braids should have dripping wet. They appeared dry. When the ranger pulled her off her horse onto the ground, her wet clothes would have caused the dry dirt to cake on her like mud.
Continuity mistake: In the ending long profile shot of old Mattie, the tree, and the tombstones, you can see that they are standing on a hill. In the following shot, you see the back of old Mattie walking away from the tombstones. The top of her head stays at a constant height while she walks quite a long distance. This would not be the case if she were working down a hill - her head would slowly get closer to the bottom of the screen.
Continuity mistake: When Mattie is trying to catch up with Jeff Bridges and Matt Damon when they initially leave town without her, she comes to the river and sees them more than 100 yards on from the river. She then drives her horse through the river and they stop and watch her. When she exits the river they're right on the river bank although they did not move throughout the scene.
Continuity mistake: There is a scene where it is pouring rain and they are all huddling under a tarp hung between trees. when they start to walk around afterwards, the ground is completely dry; no mud.