After Hud hits the guy at the bar, he takes a swig from his bottle-in-bag, which is on the bar, but which he had left at the table.
When a drunken Hud leaves his house right before he goes to attack Alma, the screen door is left wide open. After a closeup on Hud, he turns around to the house, and the screen door is now closed.
When Hud leaves the married woman's house, he puts on his boots and his pants are tucked into the boots. The next time you see him, his pants are outside of his boots.
The position of Lon's arm changes as he as Hud share a drink and talk about a girl after the pig contest.
At around 1hr 11mins, Homer comes into Hud's bedroom where he finds him lying on his bed watching television. The only light is coming from the TV, which illuminates the room. Homer turns the light on and then goes across the room to turn off the TV; no light is coming from the screen.
When Hud and Lonnie race away from Joe Scanlan's house in Hud's Cadillac the tires squeal and whine all over the road; however, the road has no pavement on it.
As Lon exits diner in opening scene, radio announcer says time is "6:26" a.m. - but lack of long shadows clearly identifies time as near noon.
The first time Hud pulls his car in at the house and brakes on the dirt/gravel driveway, the sound effect is the tires screeching. This sound happens only on asphalt or concrete, never on a low-traction surface like dirt or gravel.
The audio sound of the motor of the pickup in the opening scenes is clearly that of an inline engine. However, the GMC pickup had "V8" emblems clearly seen on the front fender sides, and the sound of a v-type engine is quite dissimilar to that of an inline.
In the first scene, you can see the cameramen and tripod of the cameras in the window of the store.
When the vet first comes and is talking to them all about possible causes of the cow death, the wind comes up and causes one of the mylar reflectors to wobble, creating a flash in the scene like someone fired a camera.