Chapter Seven: The Bite - S3-E7
Continuity mistake: At the very beginning the band the camera pans by is totally different from the one standing behind the mayor later on. For one there are no more clarinet players, a cymbal player suddenly appears right behind where the mayor stands and the drummers are 3 totally different boys, with the 3rd from the right missing his glasses suddenly and the second drummer being much shorter. (00:00:40 - 00:01:15)
Chapter Seven: The Bite - S3-E7
Revealing mistake: When Joyce and Hop get out of the car at the fair, you can see Hopper's, Smirnoff's, and Joyce's breath as if it was cold out, despite being a July 4th fair. The grass looks dead as well. (00:30:20)
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.