The doctor says "A virus is alive, it has an intelligence," neither of which is true, and he would know that. A virus is one of the simplest biological constructs we know of. It consists of the DNA for a few simple genes, all wrapped up in a protein sheath. It has no metabolism, no organs, no cell plasma or nucleus, it cannot move by itself and it has no brain or central nervous system of even the most primitive kind. It doesn't eat or drink. It can't reproduce by itself. All it can do is infect a host cell and add its DNA to the cell's existing DNA, which then produces duplicates of the virus when the cell divides. The sickness it causes it merely the side effect of the hijacking of the cell's internal duplication system, which can be severe if it attacks critical cells.
A better real-life parallel to the life form in the show is a fungus.
In the scene where Dr. Owens is explaining how a virus works to Joyce and Hopper, he describes a virus as being "alive" and having an intelligence. This is not correct, as the general consensus among biologists is that viruses are not alive. They do not have the ability to sustain themselves, create energy, grow, or have a nucleus, thus failing most of the scientific criterion for classifying something as being alive.
The package with the cassette tape that is being mailed to the Chicago Sun-Times has a 20 cent stamp on it. In 1984, that would only be enough postage for a one-ounce letter size envelope. The package is over-sized and more than one ounce. It is insufficient postage and would be returned to sender.
It is not possible to see in the dark with binoculars, such as those Lucas is using. Twilight is the best they can do.
When Nancy and Jonathan visit Murray's bunker they pass by an American flag displayed hanging vertically, however, it is hanging wrong. When displayed horizontally or vertically against a wall, the Union should be uppermost and to the flag's own right, that is, to the observer's left. (I.e. Flag should be hung with the Stars always on the left whether horizontal or vertical when you are viewing it).
Jonathan picks up an empty Polaroid cartridge, on which the words 'Impossible Impulse Battery' are just about visible. The Impossible Project (renamed Polaroid Originals in 2017) was founded in 2008 to continue supplying film for Polaroid cameras, 24 years after the year this takes place.
The Soldiers in the tunnel appear to be carrying contemporary M4 carbines, which were not introduced until 1994.
From 03:33 -apparently "Dart" can magically dig hundreds of feet of underground tunnels unaided.