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)
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).
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: 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: 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)
Other mistake: In the beginning when Kali and her gang are running from the police in Pittsburgh and she "collapses" the tunnel, her 008 tattoo is shown this way so we can read it as such, but it should in fact be read 800 to be the correct way. (00:03:15)