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 Six: E Pluribus Unum - S3-E6
Continuity mistake: When the librarian picks up the phone he picks up the one in the middle, in the next shot we see he has the one to the left and when he hangs up it's the one in the center again.
Chapter Five: The Flayed - S3-E5
Continuity mistake: When Robin takes the chemical container off Erica, in the shot of Erica saying "It could be useful", Robin is holding the container upright. In the next shot, it is being held at a rotated angle, then upright in the following shot. Two shots after that, the container is suddenly rotated again, then back to upright when the shot cuts. (00:17:25)
Chapter Five: The Flayed - S3-E5
Continuity mistake: Eleven and Mike are sitting next to each other at the hospital. Eleven looks to her left in one shot and the camera changes angles. She's now looking straight ahead and immediately looks to the left again.
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.