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 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)