When Finney breaks down crying, his hair is soaked from sweating and exertion. Seconds later when he is on the phone his hair is dry.
When Gwen jumps into the police car with Vance, the number on the rear quarter panel is 440. In the next scene when the car pulls alongside the house, the number is now 426.
Finn uses the lid from the toilet tank to break trough what seems to be a concrete basement wall. The lid is made out of ceramic and very likely would crash immediately when hitting the concrete. Also, minutes later when the hole is completed, it has become squarish which would be impossible to achieve without specialized tools.
When the father of Finney is shown reading The Rocky Mountain News, the paper is printed in broadsheet style. This is incorrect, in 1978, that newspaper was printed in tabloid format.
Finney uses what appears to be an LED flashlight during his captivity. These would not have been available in the late 70s and were not common until the 21st century.
The movie takes place in 1978, but we see the completed Wells Fargo Center in the opening credits. However, it wasn't fully built until 1983.
The opening scene in 1978 shows a guy at the kid's ballgame opening a can of Coors using a pull tab. In 1978 Coors wasn't using pull tabs but instead used it's own unique two hole push-in style opening system.
Finns flashlight is obviously an LED, which weren't available until early 2000's.
When Vance hopper is playing pinball and someone is thrown into a magazine rack, there are 4 Starlog issues seen on the magazine rack. The first one from the top is Number 17 from October 1978 the year the movie takes place. But the next three issues are Number 27 from October 1979, Number 28 from November 79 and the bottom issue is Number 31 from February 1980.
When driving through the North Denver neighborhood, one sees a rather large palm tree in a front yard. Not possible climatically in Denver.