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.
When Jack arrives at Aguirre's trailer his pickup is parked nose in, but when he and Ennis are back from the mountain and Ennis is helping him get his truck started, it is facing nose out.
Supermarket scene. An end row of bottles is hit/broken. When the camera moves back, the bottles are again upright and intact.
When Jack is in Mexico, the same boy (in a blue-striped shirt) runs behind him twice.
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.
Alma washes clothes by hand with a washboard, but she lives over a laundromat. Laundromats still cost money to use, and the Del Mars aren't particularly flush with money.
In the beginning of the movie we see Ennis is wearing his Carrhart chore jacket. The jacket in the first part is like brand new. As soon as he goes into the trailer it's worn and looks like it's a few years old. And as the movie goes on it's even more worn even though it's only been a few days or weeks.
During the hail storm scene on Brokeback Mountain, although one can hear the wind blowing very hard, the trees in the distance remain still.
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.
When Cassie dances with Ennis the first time (in the late 1970s), the jukebox is playing Steve Earle's revamped version of "The Devil's Right Hand," which first came out in 1987-8.
Early in the film (1963, Wyoming) a train is seen crossing the screen without a caboose. Cabooses were required by law on all trains into the 1980s.
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.
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.
When Jack is at the bar, turning around to eye Lureen, just before she approaches him for the first time, soft contact lens are visible in his eyes.
When Ennis tells Alma that he and Jack are going fishing, he hands Alma Jr. to her. While in her mother's arms, Junior's pajama top hikes up and you can see her microphone cord.
When they are being viewed from a far as they play with their shirts off, on the far left there is filming equipment visible within the trees.
The two teams playing football on TV during the Thanksgiving Day dinner are the CFLs Edmonton Eskimos and Montreal Allouttes. Obtaining the rights to NFL footage is very expensive for just a few seconds of background footage.
When Ennis receives Jack's first postcard it's sent general delivery because Jack didn't know Ennis's address. When Ennis writes back all he says is, "You bet," and leaves no return address, thus Jack still doesn't know where he lives (just the town). In the next scene Jack arrives at Ennis's apartment without this vital information.
Both young men, having grown up in Wyoming, would not take such a job without a sleeping bag. If you grew up in the mountains, you would know that snow is expected every month of the year and it is not uncommon for the temperature to drop to below freezing on a regular basis. A blanket or two or even three is not enough.
When Jack rides the bull (at the same rodeo where he meets Lureen), he is thrown to the ground before the eight-second buzzer sounds. This should have counted as a no-score.
On the first postcard Jack sends Ennis, he incorrectly uses "your" instead of the correct "you're" or "you are" when writing "drop me a line say if your there."