Other mistake: When Will is in the hospital, talking about being frozen, they cut to the hallway and the lights on the suspended ceiling are 2' x 4' LED lights that didn't come out until the 2000s.
Factual error: When Steve and Nancy are making out, the intro to "A Hazy Shade of Winter" by The Bangles plays. While the original version of the song was released by folk-pop duo Simon and Garfunkel was released in 1968, the version heard here was released in 1986, 3 years after the scene takes place. (00:52:30 - 00:55:40)
Continuity mistake: When Will is being chased by the monster and runs into his garage, he pulls down his .22 rifle hanging on a wall rack. After he loads a few rounds aiming it at the door, the monster appears behind him. After Will and the monster disappear, the scene shows his rifle is back on the wall rack in its original spot. Did Will or the monster place it back on the wall rack while Will was being abducted? (00:07:25 - 00:08:10)
Factual error: When Nancy shoots the creature the second time, she fires at least 8 shots from a 6-shot revolver without reloading. And that's assuming she reloaded after firing 3 shots at the creature the first time, even though the situation made it seem unlikely. (00:20:30)
Factual error: When Dustin's toy collection "comes to life" (thanks to Eleven), one of the robots is the Transformer Ultra Magnus. Though it appears to be modified with a motorized engine that the actual toy didn't have, that's not the mistake, as there no reason Dustin couldn't have modified it. The error is that the season is set in July 1985, but Ultra Magnus wasn't released in the U.S. until 1986. (Yes, the toy was available as a Diaclone in Japan, but in a different color scheme. Dustin's has the coloring of the U.S. version, and even if he somehow ended up with the Diaclone version, it's highly unlikely he would have repainted a Japanese toy to resemble a Transformer he wouldn't know about the existence of yet, as the 'Transformers' animated movie wasn't released until August 1986).
Continuity mistake: In episode 1, during Will's appointment discussing his episode, the doctor said "how did you feel, when you saw the storm?" Will then replies "Frozen." Doctor says "Heart racing?" Will: "Just frozen." Doctor: "Frozen? Cold frozen? Frozen to the touch?" In this episode When the doctor watches the tape of his conversation with Will from the previous episode the dialogue is different. On the tape, after Will says "frozen" the doctor then says "actually frozen cold?" Which he didn't say in the previous episode. And his actual lines from the first episode are missing. (00:25:15)
Continuity mistake: When Hopper is taking Todd's car at the gas station he hands Todd a Slim Jim he was eating, then they pan away from them to show Joyce Byers and Hop talking. When the scene widens Hop has the Slim Jim back in his hand and hands it to Todd again. (00:24:25)
Factual error: Joyce asks Will if he remembers when he was 8 and she bought him that giant box of crayons for his birthday - the 120 pack. Will is 13 in the show, so when he was 8, it was 1979. There were only 78 colors available in 1979 and the 120 crayons box was not available until 1998, almost 20 years later.
Continuity mistake: While Hopper lies his way into stealing the car from Todd and claims that Alexei has murdered many children, Hopper pushes the driver seat back into its proper upright position. Then as Hopper introduces Detective Byers to Todd, the driver seat is once again leaning toward the steering wheel before being upright in the next shot. (00:24:30)
Factual error: When there is a pan closeup shot of the walkie-talkie the boys are going to finally talk to with the sheriff, there is a book called Great American Ghost Stories by Hans Holzer on the shelf to the left. That book was published in 1990, not in 1983, the year of the episode. (00:18:30)