Chapter Seven: The Massacre at Hawkins Lab - S4-E7
Other mistake: When Hooper was trying to light the alcohol-soaked rag to make a torch, his lighter wasn't working at first. He finally got a flame at the last minute. However, the sparks alone should have been enough to ignite the flammable rag. (01:00:33 - 01:02:03)
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.