The boy's banjo is an open-back model used in old-time music, and he plays "claw-hammer" or "drop-thumb" style. During "Dueling Banjos", he plays a resonator banjo, with finger picks, in the more modern three-finger bluegrass style.
When the main characters first meet the hillbillies they're on the right side of the river. After the accident in the rapids, and for Ed's climb up the cliff, they're on the left side of the river. They never passed under a bridge, but the surviving hillbilly is on that side of the river with them.
After Lewis and Drew hire the Griner brothers to drive their cars down to Aintry, the weather shifts from overcast to bright and sunny a few seconds after they leave the Griner's garage. It cycles rapidly between overcast and sunny while paddling in the same section of the river.
During the drive to the river near the start of the film, trees and other background items are reflected in the windshield. They should've been blocked by the canoes on the roof (and they are in the long shots).
When the men bury the first body, the 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 adjust their grip on the rock, and his arms move slightly.