Chapter Eight: The Mind Flayer - S2-E8
Factual error: When El arrives, Hopper rifle has a picatinny rail, inaccurate for the time period.
Factual error: During the police pursuit in Pittsburgh, the officers are wearing shoulder speaker/microphone handsets. The models are the Motorola Saber radio, which wasn't introduced until 1989, while the season takes place in 1984.
Chapter Nine: The Gate - S2-E9
Factual error: The military convoy has Desert Tan HMMWVs (Humvees). The Army didn't take delivery of the first ones until 1985 and they certainly were not painted Desert Tan until 1991 in the Gulf War.
Chapter Eight: The Mind Flayer - S2-E8
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.
Chapter Three: The Pollywog - S2-E3
Factual error: The cordless drill used to fix door bolts in the new cabin is modern, not from the 80s.
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.