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)
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.
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.