Continuity mistake: Watch carefully in the scene where the four boys are running for their lives over the railway bridge. Chris and Teddy are in front, and Vern and Gordie are behind. In one shot we see just Vern and Gordie's feet, and quite a lot of the track behind them. You can see that there is no train behind them. In the very next shot, the train is about two metres behind them.
Continuity mistake: When the boys are roasting marshmallows, Verns falls off the stick in two pieces. When he picks it back up with the stick, it's whole again.
Revealing mistake: When Gordie cries after finding the body there are no tears in his eyes and his face is completely dry. (01:10:45)
Answer: A cello.