Chapter Four: The Body - S1-E4
Continuity mistake: Right after Nancy puts together the torn photo of Barb at the pool it cuts to Dr. Brennan and crew in hazmat suits. At the beginning of the shot when they try to reach Shepard on the comms, the digital clock reads 6:54:08. After Brennan says "Try him again.", the shot has the clock blurred in the background as 6:40 something, and a bit later it reads 6:40:55. (00:27:20 - 00:27:54)
Chapter Seven: The Massacre at Hawkins Lab - S4-E7
Continuity mistake: Eddie is still wearing a denim vest he took off his body and threw at Steve a few seconds earlier. When the camera cuts away and back, the vest is gone again. (00:15:19)
Chapter One: Suzie, Do You Copy? - S3-E1
Continuity mistake: In Bruce's hamburger the salad is on the bottom and the bacon is on the top, in the next shot the salad is on the top and the bacon has disappeared, replaced with ketchup. (00:23:05 - 00:27:10)
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.