When Ennis was driving Alma Jr. home from the bar where Alma and his waitress girlfriend have met, along the path in the background, next to the Dodge Neon, is a RV or trailer car. These had not been introduced yet.
Leading up to the argument that sparks the Del Mar divorce (finalized on 6 November 1975), Ennis is on the couch watching Kojak (1973) on a Saturday night. Kojak didn't air on Saturday nights until its 5th season, 1977-8.
Ennis gets a postcard from Jack which reads, "Ennis, see you in a couple weeks, fish should be jumping. Jack." The postmark is Childress, Texas, July 1972. A couple of weeks later, Jack is driving a blue Ford truck with a 1976-7 grill.
The type of fireworks display in the 4th of July scene was unheard of in the United States during the 1960s. At that time, almost all fireworks used in the U.S. were made in the U.S., and were generally fired one at a time. Chinese-made fireworks did not become prevalent in the United States until after trade relations were opened up with China in 1972. The low-level Chinese-style fireworks shown in the film were not imported into the United States during the 1960s and did not become popular in fireworks displays in the U.S. until the 1990s at the earliest.
During the scene in the supermarket where Alma works in the 1960s, plastic cranberry juice bottles with yellow, plastic twist lids on a shelf are visible behind Ennis' head. These bottles would have been glass until the early 1990's.
The first reunion of Jack and Ennis is on 24th September 1967. On the next day 25th, they went onto the mountains and at that night there was a full moon. However, the full moon of September 1967 should be at 18th day, not 24th or 25th.
Jack is chopping wood when Aguirre approaches to give news of his sick uncle. Jack places the next log on the chopping block as he turns to speak to Aguirre. After the conversation is over and Jack turns back to chopping, the log is gone and he replaces the log that was never chopped.
During Jack's first and second bull rides, his free hand alternates several times from being his left or right hand. Since he's obviously not letting go to switch hands, either some shots are flipped or the sequence is edited from several takes.
Ennis, standing in the river washing a coffee pot with a rag, pauses to look up and spots Jack on horseback in the distance on a mountainside. Ennis then slings the washrag over his right shoulder and starts to exit the river. In the next scene, as he exits the river onto the riverbank, the washrag is gone from his shoulder and back to being held in his right hand.
When Jack rides into camp complaining that he is "commutin' four hours a day" he takes off one glove, the one on his left hand, to feel the coffee pot. It then shows him drinking coffee with his left hand and the right hand glove is still on and he is holding the glove from the left hand with his right hand. When Ennis says (the second time) "I wouldn't mind being out there" it shows Jack with the coffee cup still in his left hand but the glove from his right hand is now off.
At the beginning of the movie where we first see Jack Twist driving up in his truck, the window on the passenger side is up as evidenced by the reflections off the glass. Then as the truck turns into the parking lot, the same window is now down.
Towards the end of the movie when Alma Jr. visits her dad in his trailer, he goes to the cupboard to get them a drink and cups. The cupboard door at Ennis' head is seen alternately opened and closed without him touching it.
In the scene where Jack Twist is first introduced to Alma by Ennis, her purse is on her left shoulder and her arms are down - immediately in the next scene when she is shown, her arms are folded across her chest in a "defensive" stance.
Enis is helping Jack start his truck after they get back from Brokeback Mt. As soon as the truck starts Enis slams the hood down, but it is not all the way closed. Enis then leans on the truck and he and Jack have a conversation. In the next scene you see Jack driving the truck away and the hood is completely closed.
The two teams playing football on TV during the Thanksgiving Day dinner are the NY Jets and the Pittsburgh Steelers. Only the Detroit Lions and Dallas Cowboys hosted Thanksgiving Day games at the time.
During the rather important kitchen sink conversations after the family holiday dinner, a props tag is plainly visible on a dish's underside. As the dish is conspicuously hand-washed under running water, the underside faces the camera and the large masking tape tag with marker lettering goes unacknowledged by the cast.
When Ennis comes back to camp late at night after he encounters the bear, Jack asks him where he's been. It's supposed to be cold but when Jack talks you can't see his breath. When Ennis tells his story about the bear, you can see his breath, but when they continue talking, neither one's breath can be seen in the cold night air.