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)
Deliberate mistake: Stranger Things season 4 vol 1 confirms upside down is locked in 1983 as Nancy's guns aren't there. Her diary entry shows it to be Nov 1983. Yet in season 4 vol 2 Ed Munson's guitar is there. This implies he either was playing guitar back then or they've made a deliberate mistake here but it's caused an error in the 'stuck in 1983' story.
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.
Factual error: Because of the way radios work, when Suzie and Dustin are singing their duet they wouldn't be able to hear each other and the rest of the party would only hear a garbled mess. Per frequency only one radio should transmit at each time, so all the others can receive. If multiple radios are transmitting simultaneously, the signals will overlap and result in unintelligible noise. (00:35:53)