Chapter One: The Hellfire Club - S4-E1
Continuity mistake: Tammy Thompson's voice changes octaves as she sings the National Anthem. At the start, she's in a normal mezzo range. When the camera cuts from her to the back-and-forth dialogue between Steve, Robin, and Vickie, her voice then changes downscale to a very noticeable alto.
Chapter Two: Vecna's Curse - S4-E2
Continuity mistake: When El advances on Angela and her friends at the skating rink and wallops Angela in the face with the skate, her aim is upwards at the lower cusp of Angela's nose for the impact. When Angela's friends sit her up in the following shot, the area between her eyes is cracked and bleeding instead as she screams in agony.
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 Five: The Nina Project - S4-E5
Continuity mistake: The seat belt keeps appearing and reappearing on Nancy's shoulder as she's driving.
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 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)
Chapter Seven: The Massacre at Hawkins Lab - S4-E7
Plot hole: The upside down should be mirrored, i.e. all left is right and right is left. When the team finds a gateway where Chrissy died, they should be seeing a mirror image through the ceiling. Also, all buildings should have been reversed. Directions to get there on the bike would have been confusing and reversed, and when any writing was seen in the upside down, it would have been backward.
Continuity mistake: Eleven makes the helicopter fall from the sky and blow up some army vehicles. Within a cut, it's much earlier in the day as the sun is right above her. Prior to this, it was lower in the sky and returns that way as she and the rest of the crew are driving away.
Answer: This is answered in a later episode so you should watch the entire season, but if you really want to know...SPOILERS She didn't, 1 (Vecna) killed all those children.
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