Chapter Eight: The Battle of Starcourt - S3-E8
Factual error: The value of Planck's constant used in the show, 6.62607004 is the value that was measured in 2014. The value that was in use in 1985 was 6.626176. The value was updated again in 2018, to 6.62607015.
Chapter One: Suzie, Do You Copy? - S3-E1
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).
Chapter One: The Vanishing of Will Byers - S1-E1
Factual error: The boys need Will to roll a 13 to hit the Demogorgon with the fireball in the DnD game. Fireball is an automatic hit. Players roll for damage only. They mention the 13 needed to hit in the final episode as well.
Chapter Five: The Flea and the Acrobat - S1-E5
Factual error: The bus that appears in the scene is a Blue Bird TC2000 dating from 1998 to 2003, long after the setting of the series. (00:38:15)
Chapter Two: Trick or Treat, Freak - S2-E2
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)
Chapter Eight: The Upside Down - S1-E8
Factual error: This error occurs multiple times during the Series but most after 11 and the demogorgon disappear from the class room a modern periodic table is shown. There's at least a 1/2 dozen elements on it that weren't discovered until the 2000's.
Chapter Seven: The Bathtub - S1-E7
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)
Chapter Two: The Mall Rats - S3-E2
Continuity mistake: Steve throws his empty banana peel on the table in the back of the ice cream store, then is shown seconds later still holding the empty peel.
Chapter Five: The Flayed - S3-E5
Factual error: When Hopper steals the yellow convertible at the gas station mini mart, the gas pumps have vapor recovery boots on the nozzles. Those weren't in use for another half decade.
Chapter Five: The Flayed - S3-E5
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)
Chapter Five: The Flayed - S3-E5
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)
Chapter Six: E Pluribus Unum - S3-E6
Factual error: When the Russian requests the cherry flavor and refuses to talk, he gets thrown out. At the back is an oil drum from the French energy company "Total." However that logo was introduced only in the mid 2000s. (00:11:55)
Chapter Five: The Flayed - S3-E5
Visible crew/equipment: While Bruce chases after Nancy and Jonathan they hide in the OR, then Nancy picks up the red phone and shouts, and in the following closeup of the ceiling mounted domed light, the upside-down reflection of a crew member wearing shorts and sneakers can be seen quickly. A few minutes later, after Jonathan jabs scissors into Tom's throat, when Jonathan leans back out-of-breath there's another closeup of the surgical light, and we can see the upside-down reflection of the cameraman's arm and part of the camera as it moves closer, then it's visible again a few shots later. (00:43:10 - 00:46:40)
Chapter Seven: The Bathtub - S1-E7
Factual error: High school basket ball courts didn't have a three point line until the 1987-88 season - not in 1983.
Chapter Three: Holly, Jolly - S1-E3
Continuity mistake: When Mrs. Wheeler bursts through the door, angry with Nancy about lying to the police she throws her purse down. When they get to the kitchen she throws the same purse down again.
Chapter One: Suzie, Do You Copy? - S3-E1
Continuity mistake: When Billy crashes his car, it stops and breaks down. He moves to the bonnet and a clear hissing from a damaged is audible. Then all the mess happens and the episode ends with the usual cliffhanger. When the second episode starts, Billy's car battery is still up, even with the headlight turned on, and the car starts at once with no problem whatsoever.
Chapter Four: The Body - S1-E4
Factual error: When Chief Hopper cuts open the fake body of Will, it appears that he opens a knife with a thumb assisted blade which wasn't invented until 1995.
Chapter Four: The Body - S1-E4
Continuity mistake: Right after Nancy puts together the torn photo of Barb at the pool it cuts to Dr. Brennan and crew in hazmat suits. At the beginning of the shot when they try to reach Shepard on the comms, the digital clock reads 6:54:08. After Brennan says "Try him again.", the shot has the clock blurred in the background as 6:40 something, and a bit later it reads 6:40:55. (00:27:20 - 00:27:54)
Chapter Six: The Monster - S1-E6
Continuity mistake: Eleven is in the shop and stealing eggos. As the freezer door shuts she walks away holding three boxes. The next shot of her walking towards the exit she is holding four.
Chapter Five: The Flayed - S3-E5
Factual error: When Eleven is handed M&Ms in the hospital she is given green and red. Red M&Ms were not reintroduced until 1987. (00:42:28)
Answer: The body was fake, made by the scientists at Hawkins Lab in an attempt to cover up his disappearance and the existence of the Upside Down.