During several of the nighttime searches, characters use flashlights with modern bulbs (identifiable by their blue-white color and visibly different from the yellow-orange hue of period-correct flashlights). Ironically, if they had all used modern bulbs, the error wouldn't be nearly so obvious.
The teenage characters frequently use the phrase "chill" to mean hang around. In 1983, "chill" would have been used as a command to someone the speaker perceived to be overexcited about something. "Chill" didn't mean "hang out" until some point in the 1990's.
The story line predates many of the vehicles. The police chief drives a K5 Blazer with a facelift package which didn't come out until the next year, the buses all appear to be from the 90s - especially the bus on the International chassis near the end of Episode 8, which clearly has the 90s/2000s "anteater" style hood.
The police light-bar on the Chief Hopper's 1980 Blazer is a Code 3 MX7000, manufactured by Public Safety Equipment, which was not sold until the mid-1990's.
While the HK MP-5K submachinegun carried by the Chief and the MPs/bad men existed as early as 1976 several characters are seen carrying MP-5K PDWs (identified by an attached sidefolding stock) which was not introduced until 1991.
A number of times you can see the bad guys using Crosman 2240 airguns but they were not introduced until 1999.