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.
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.