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 Angela's nose for the impact. When Angela's friends sit her up in the following shot, the upper bridge of her nose between her eyes is cracked and bleeding 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.