Revealing mistake: After they leave the fort, in the forest glade when the Huron start hollering, the air above the soldiers is very clear. But in the next shot just when the guns start to fire, there is a smoky haze hanging over the forest. Apparently the first attempt to film this scene didn't make it to film and some of the residual smoke was still there when they filmed it the second time.
Revealing mistake: When Magua hits the British soldier with his axe before the ambush, it's quite obvious the soldier has been replaced by a dummy. (00:17:10)
Revealing mistake: When Chingachgook throws his axe in the back of the Huron we see the Huron lying on his stomach with the axe stuck to his back. When the camera moves up leaving the Huron out of sight, we see Chingachgook picking the axe up from a total different angle than the axe stuck to the Huron. (00:19:45)
Revealing mistake: When Uncas is being murdered and thrown down, Daniel Day Lewis comes running from between the rocks. He then hits the rocks on the viewer's left side. You can see them moving since they are made of linen.
Revealing mistake: Uncas dies and falls off the cliffside. Immediately after this happens, Hawkeye comes into view through a passage in the rocks. If you look at the rock to Hawkeye's left, when he touches it with his hand, it flexes.
Answer: Major Hayward is supporting one of the soldiers as they enter the cave. After they make their way down the rocks, he puts the soldier down on the ground. The other soldier appears to be killed by one of Magua's gang.