Chapter Seven: The Massacre at Hawkins Lab - S4-E7
Continuity mistake: Eddie is still wearing a denim vest he took off his body and threw at Steve a few seconds earlier. When the camera cuts away and back, the vest is gone again. (00:15:19)
Chapter Five: The Nina Project - S4-E5
Continuity mistake: The seat belt keeps appearing and reappearing on Nancy's shoulder as she's driving.
Continuity mistake: Eleven makes the helicopter fall from the sky and blow up some army vehicles. Within a cut, it's much earlier in the day as the sun is right above her. Prior to this, it was lower in the sky and returns that way as she and the rest of the crew are driving away.
Chapter Two: Vecna's Curse - S4-E2
Continuity mistake: After Mike kisses El on the forehead, Jonathan is in the back grabbing his glasses from his pocket. In one shot, he's grabbing them out, and then the next shot has them at waist-level.
Chapter Two: Vecna's Curse - S4-E2
Continuity mistake: When El advances on Angela and her friends at the skating rink and wallops Angela in the face with the skate, her aim is upwards at the lower cusp of Angela's nose for the impact. When Angela's friends sit her up in the following shot, the area between her eyes is cracked and bleeding instead as she screams in agony.
Chapter One: The Hellfire Club - S4-E1
Continuity mistake: Tammy Thompson's voice changes octaves as she sings the National Anthem. At the start, she's in a normal mezzo range. When the camera cuts from her to the back-and-forth dialogue between Steve, Robin, and Vickie, her voice then changes downscale to a very noticeable alto.
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.