Yaphet Kotto looks through the 1/3 open blinds and watches Redford test the electric 'torture' box on himself.... But immediately before & after, when he's in the office talking w/ Redford, the blinds on both sides of the door are totally closed.
When C.P. 'Woody' Woodward (M. Emmet Walsh) gets out of his car with the chocolate prune cake wrapped in aluminum foil, the height is considerably lower and the foil is irregular on top, like a pie, than later in Brubaker's office when the foil takes on the shape of a cake and is considerably higher.
When Brubaker is eating a '70s-style TV dinner, multiple items change position/orientation in successive cuts. Specifically, the Tar-Tar sauce jar (closed/opened) and the bag of vegetables (changes position).
When Brubaker & another prisoner are taken to a local restaurant to drop off a steer; the guard drinks a beer which changes position in his hand after the camera goes back to the store owner for less than a second or two.
Brubaker takes the (hot?) TV-Dinner out of the oven with his bare hands and carries it casually to the dining room table.
As Warden Renfro leaves, he tells Brubaker, "Smartest thing you ever did was come in here 'incognizant'; dumbest was stepping forward." The term is "incogNITO", which means in disguise or unrecognizable. "Incognizant" means ignorant.
When Warden Henry Brubaker and Lillian Gray are drinking Budweiser Beer out of a can, the can should be steel on the bottom and sides and not aluminum since the movie is set in 1969.
At the end, when the car is leaving the prison, the grass around is violently shaking revealing the helicopter carrying the camera.
The movie supposedly takes place in Arkansas (although this is never explicitly stated, it is clearly indicated to be in the South, and likely near Texas and Louisiana). However, on the vehicles you can clearly see the OHIO license plates displayed. The movie was filmed about 3 miles west of Junction City, Ohio.
In a restaurant scene, a Conn's Potato Chip sales display can be seen. Conn's are only made and sold in Ohio, not Arkansas.
Brubaker finds a maggot in his plate of beans. Another prisoner suggests that he should eat it as it is 'full of protein'. However, the beans themselves are high in protein and, in comparison, the contribution from the worm to his diet would be negligible.