At the beginning of the story the dark-suited man pulls up and stops his 1949 Buick. The car that later goes over the cliff is a 1950 model.
Janice Kendon tells the deputy that the price of gold is $15 an ounce more in Mexico than in the United States, or a difference of $2400 per 100 pounds. The actual difference would be $240,000 per 100 pounds, providing a much greater motive for the murders that occur.
The helicopter pursuing the car in the climactic chase is shown moving at a high speed, but in close-ups of the deputy and pilot in the copter, the clouds behind are stationary and there's no sign of movement at all.
Shots of the County Courthouse show that the correct spelling of the actual Arizona county where the action is set is "Mohave". However, on one of the teletypes coming into police headquarters, the county's name is spelled "Mojave", the Spanish spelling.
Possible spoiler. Near the end, when Les reaches the plane with his car after being shot at, as he exits his car, the reflection of two crew members can be seen on the car window.
After they've retrieved Charley's body from the canyon, the camera pans on the seaplane taking off from the river and the shadows of the camera crew are clearly visible on a boulder in the foreground.
The deputy does not drink when offered at the saloon because he's on duty and seeking information but when taking the drunk Kendon son home from that bar he accepts a beer from the guy's dad while presumably still on duty.