At the dedication of Buddy Deeds' plaque, it is supposed to be midday (Pilar says she is on lunch break), but long shadows are visible. A few minutes later, by the river, the sun is back overhead.
In the bar, the head of the beer in the jug in the background repeatedly changes size between shots.
When Pvt Johnson is told she failed the drug test, the rank on her hat is upside down.
When COL Payne is reprimanding PVT Johnson about her failing the drug test, she has her hat on. Members of the US Army do not wear hats indoors. When he's finished he salutes her and she returns his salute--that's backwards. She should salute him first and hold the salute until he returns it.
Sheriff Charlie Wade is armed with a Colt 'New Frontier' revolver, first introduced in 1961 by Colt as an updated version of the old Colt 'Peacemaker'. The events in the film take place in 1957 and 1958.
When Otis draws a beer he pours it straight-in, creating a large head. The correct way is to hold the glass at an angle, which creates a smaller head. A bar owner (particularly one who has been at it for over thirty years) would know this.
Mercedes dials a three digit number to call the border patrol. While some might assume she was calling 911, which does not directly reach the border patrol, with the frequency of people crossing the river through her property, it is likely that she has the correct number on speed dial.
When Mikey and Cliff are in Mikey's sculpture studio, around 55 minutes in, Cliff holds up a fired .45 bullet that had clearly pancaked against a smooth, flat surface and he asks, "What's a pistol slug doing on a rifle range?" While pistols are also fired at outdoor ranges, their limited range requires the targets to be closer in than those on a rifle range. In a location such as the one shown there is plenty of space for separate pistol and rifle ranges.
When Col. Payne enters Big O's bar, the birds (Eagles denoting full colonel rank) on his shoulders are upside down.
After Charlie has shot the Mexican on the bridge and bends over his body, the Mexican slightly moves.
The license plate on the car at the drive-in is for 1972 whereas the movie was released in 1973.
The location of the abandoned drive-in is flat whereas real drive-ins had rows of banked earth so that cars could allow for backseat viewing by rolling forward or backward to achieve the ideal angle.
A woman mentions that a high school football player runs the 40-yard dash in 3.4 seconds. This is highly unlikely, as most of the records for the fastest 40-yard dash are around 4 seconds flat.