Chapter Eight: The Upside Down - S1-E8
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)
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.
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.