When the kidnappers are driving into the onion field, there is a sign advertising a motel 15 miles south on Hwy. 166. This would not be possible because Hwy. 166 only runs exactly east and west.
In the beginning of the film (which takes place in 1963), as the camera pans down the street, past houses, a 1965 Chevy Impala is seen parked in one of the driveways.
Some of the clothing the characters are wearing (and most of the extras) are more modern late 70's style and not the appropriate 1963 style.
When Jimmy is seen watching television at Greg's house he is watching "General Hospital". This scene takes place in early March 1963 while "General Hospital" debuted April 1, 1963.
At around the eight-minute mark when the two police officers are in the diner, a neon sign behind them advertises a slice of pizza for fifty-cents. Back in 1963 fifty-cents would be the equivalent of a little more than $4.50 in 2021 which would have been unheard of back then. But at the time of filming, it equates to about $2.00 in 2021 which is more reasonable. Same goes for the $14.95 price of a wash and wax at the car wash in 1963 - which equates to over $136 in 2021, but from the time of the film it's a more reasonable $60.
During the kidnapping scene, highway speeds were mandated to 55 mph 10 years later in 1973. The speed limit sign says 55 in 1963.
When Karl Hettinger is talking into the patrol car microphone, the microphone is turned around and he is actually talking into the back of it.
When Powell drags Officer Campbell's body over to the ditch, Ted Danson keeps his arms stiff, held out from his body as if rigor mortis has set in. This would not happen to a dead body for hours.