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: 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 One: Suzie, Do You Copy? - S3-E1
Continuity mistake: In Bruce's hamburger the salad is on the bottom and the bacon is on the top, in the next shot the salad is on the top and the bacon has disappeared, replaced with ketchup. (00:23:05 - 00:27:10)
Chapter One: Suzie, Do You Copy? - S3-E1
Continuity mistake: Steve is talking to Robin about striking out. Two girls come in and Robin encourages him to talk to them. In the first angle his shirt is lower and you can see his chest hair. Cut to the other side as he flips around to talk to the girls, and his shirt has moved up. (00:24:15)
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.