When the boys and El are hiding in the gym, sunlight can be seen coming from the windows on the roof down onto the floor, despite it being nighttime in the story.
When Tommy hands Steve a can of Coke outside of the convenience store, the logo on the can changes back-and-forth between "Coke" and "Coca-Cola" in different camera angles.
When Steve holds the soda can to his head, you can clearly see that the soda can is still closed. When Steve stands up and walks to the door of the car and is confronted by Tommy, you can see that the can is opened. Also, when Steve drops the soda can to the ground, you can clearly hear that the can is empty.
At around 25:18, Mr. Clarke is watching The Thing (1982) on TV. The phone rings and he goes to pick it up. While he is talking, we hear audio from the film in the background from a part that occurs before the clip shown.
Actual de-icing salt is not cooking salt. It is raw and unprocessed, containing a significant portion of dirt and even gravel. It would yield a muddy, brownish solution and be rather disgusting for a person to immerse themselves in.
When Lucas yells the temperature corrections across the gym, the pool is already nearly full. You'd be hard pressed to affect the temperature much at that late stage.
While the group is attempting to create a sensory-deprivation chamber, they dissolve 1500 pounds of salt into a kiddie pool of the time period.
To test the specific gravity of the salted water, they see how well an egg floats. During the first test, the egg sinks. During the second test, the egg floats with a large portion sitting above the water line. Were this a real egg, it would be impossible to float that high on top of the water. But as the egg turns on the water, the hole the crew used to remove the insides of the egg can be seen.
Additionally, the pool holds roughly 100 gallons, or 800 pounds, of water (if the pool is 8 feet in diameter and the water is 2 feet deep). Even under boiling conditions, it would be impossible to dissolve much more than about 320 pounds of salt.
Additionally, the pool holds roughly 100 gallons, or 800 pounds, of water (if the pool is 8 feet in diameter and the water is 2 feet deep). Even under boiling conditions, it would be impossible to dissolve much more than about 320 pounds of salt.
When Dustin and Lucas are seen dragging the mini pool into the Hawkins Middle School gymnasium, outlines for a basketball court (more specifically a 3 point line) can be seen. The NBA didn't adopt the 3 point line until 1979, nor did the NCAA until 1986, with high school basketball following a year later. This is set in 1983, therefore there shouldn't have been a 3 point line in the gymnasium.
True sensory deprivation tanks use Epsom salts, not regular or de-icing salt, in order to provide the correct environment for the experience.
When Nancy is sitting in the school hallway outside of the gym (under the tiger mural) and Jonathan comes out to sit beside her, in the background of the wide shot you can see heavy cables and extension cords coming out of a mechanical room door, running across the hall, and going into a different set of doors.
This is not a goof, those "cables" are actually the two hoses they used to fill the kiddie pool. They run from the room with the sink to the gym via the swing doors of the gym.
The street sign embedded in the sidewalk outside the theater saying "OAK STREET" is upside down. The sign should be installed so that you can see what street you are about to cross, not the one you had just crossed (with your back to the street).
Dustin makes a call to Mr. Clarke asking how to make a sensory deprivation tank. The audio of Mr. Clarke's side of the call is made up of clips of sound from Chapter Five: The Flea and the Acrobat (2016), where the boys ask him how to enter another dimension.
When Mike answers Chief Hopper on the walkie-talkie, the radio at the Byers house is sitting on a mantel next to a book called "Great American Ghost Stories" by Hans Holzer. This book wasn't published until 1990, seven years after the story takes place.
The movie that the science teacher is watching at home with his girlfriend (The Thing (1982)) was not released on video tape until 1992.