The amount of whisky in the bottle at Lemuel's varies depending upon whether the shot is from behind Holly or from behind Garrett.
As Pat leaves Fort Sumner, a young boy throws stones at him. In longer shots, Pat and the boy are in shadow, but the close shots of the boy show him in bright sunshine with clear sharp shadows defined.
When Alias tells Billy "He 's coming in", the position of Billy's hands and the way he is holding the glass change between shots.
When Cody is lying on a bed, his left arm is over the rifle, with his hand hanging from the bed, and his right hand is leaning on his left shoulder. In the next shot, when Poe touches him, his left hand is holding the rifle and his right hand leaning on the bed.
In 1881, while Pat Garrett and his posse are shooting at Billy and his gang, who are holed up in a remote stone building, Garrett calls to Billy and says that he is wanted for the killing of Buckshot Roberts. Billy yells back that the Roberts shooting had taken place a year ago. In fact, Roberts was shot and killed in 1878--three years earlier--by Charley Bowdre, another member of Billy's gang.
Pat Garrett was killed in 1908, not 1909.
At about 1:32:00, when the ladies come into Garrett's room, one of them puts an hourglass down on the bed table. She turns it upside-down, so that all the sand is in the bottom globe.
At about 1:32:00, when the ladies are helping Garrett undress, you can see an adhesive bandage partway above belt line on his right hip.
Though the film takes place in 1881, many of the characters use Winchester Model 1892 rifles which were not available until 1892.
(at around 1 min) During the Governor's interview with Pat Garrett at the dining table, the shadow of the boom operator is clearly visible on the table cloth to the right of the Governor.
Pat Garrett says he wants to grow gray. However his hair is already gray.