In the ending sequence after the VW bus takes a right turn to set off to the open road, a modern sleek looking 1990s era Chevy Astro van is seen approaching in the opposing lane. The usual method taken by the production for open street scenes was to use a short lens to place out of time vehicles out of focus from being recognized.
The little green "Cosmic Cutie" from "The Hitchhiker's Guide to the Galaxy" wouldn't have been painted on the side of the Dead Head van as early as 1980 or 1981 when the show takes place. The original novel was published in 1979 without the famous icon on the cover, which was introduced in a later cover version.
When Nick and Sarah are practicing in Nick's basement, there is a poster on a door of a zoom into the Mandelbrot set fractal. The first computer generated fractal was crudely visualized in 1980 by Benoit Mandelbrot at IBM's research center in New York. The type of image that is displayed wouldn't have been commercially available until the 90s.
When Daniel is setting up the movie in the English class, the poster of books behind him shows "The Color Purple". This book wasn't written until 1982.
Harris is seen using an IBM PC in this episode, which takes place towards the end of the school year in the spring of 1981. The IBM PC wasn't released until August 1981.