When the men fall into the river, nobody is clinging to their paddle as instructed to. After the three survivors find the intact canoe, they still have three paddles. And the canoe is suddenly only half-filled with water.
Lewis' position in the canoe on the river while he's lying between Ed and Bobby (Lewis has a fractured femur). One long shot only shows the two paddlers while closer shots show Lewis' head in plain view, both while Lewis was on his back and propped up behind Bobby.
During the drive to the river near the start of the film, the reflections of the trees etc. can be seen in the windscreen - but this should have been blocked by the canoes on the roof (and indeed they are in the long shots of the same journey)
After Lewis and Drew hire the Griner brothers to drive their cars down to Aintry, the weather shifts from overcast to bright and sunny only a few seconds after they leave the Griner's garage, and several more times between overcast and sunny while in the same section paddling on the river.
In the final rafting scene when Ed, Bobby and Lewis are coming down together in one canoe in the last set of rapids, Lewis is lying prone in the middle of the raft, but in some scenes you can see he's not there.
The boy's banjo is an open-back model used in old-time music, and we see him playing without picks in the style known as claw-hammer, or drop-thumb. However, the famous "Dueling Banjos" tune we hear during the scene is clearly played on a resonator banjo using finger-picks, in the more modern three-finger bluegrass style.
When the men bury the first body, the "dead" man's chest briefly rises and falls like he's breathing. Later, when Ed pushes the second body into the river, the "dead" man's fingers can be seen adjusting their grip on the rock, and his arms move slightly too.