Other mistake: In the scene where Norman, his girlfriend and her family are seeing off her brother at the train station, you can see modern tractor trailers on the interstate highway in the background.
Revealing mistake: When Norman MacLean drops off Neal Burns after he falls asleep naked in the sun, Neal gets out of the car still naked with his clothes bundled in front of him. The camera angle switches to the rear view and the sunburn makeup on his back makes it look like he was wearing a bikini. His butt and back must have touched the car seat and rubbed off the makeup.
Jessie Burns: If he came back next summer, would you try and help him?
Norman Maclean: If you wanted me to.
Jessie Burns: Well, he's not coming back.
Norman Maclean: Well, at least, he's got friends out there.
Jessie Burns: Who Ronald Coleman? Why is it the people who need the most help... won't take it?
Norman Maclean: I don't know, Jess.