Jason cocks the revolver two times without firing the gun in between.
Eleven draws a stick figure diagram in the dust of the Surfer Boy Pizza van window, which changes appearance between the following shots.
Mike prepares some "pizza goggles" for Eleven to wear in the sensory deprivation bath. Mike and Eleven are talking and then Argyle comes and drops a pineapple pizza on the table for them. Between each angle, these "pizza goggles" alternate from either sitting downright (normal) on the table, but in closeup angles they are resting skyward with the arms pointing up.
When Nancy's group is released from the vines, they all pick up their weapons. Robin picks up the bag with the Molotov cocktails in it and pulls one of them out. In the next shot, at the top of the stairs, she pulls the same Molotov cocktail out again.
Near the end, Vickie walks to the table while holding a jar of peanut butter and the lid is on even after she places it on the table. Cut to a different angle and the lid is off without seeing its removal.
A flaming Molotov cocktail does not explode when the fire reaches the fuel, since there is no oxygen inside. It explodes when the bottle is shattered, spraying a fire/fuel mixture into the air. However, it requires a hard rigid object such as stone or iron to instantly fragment the glass, so even with a muscular demon the bottle will just bounce off.
When Max, Lucas, and Erica are walking through the murder house, they each have a lantern. These are actually AC-powered bug zappers that would need an extension cord.
It is unlikely that the trash can cover, even with nails punched through it, could hold in place and mostly withstand the force of the bat-like creatures outside.
When Murray engulfs all the monsters with the flamethrower, there is a metal sword on the ground which Hopper then picks up. As it is made entirely from metal, it would've been red hot and he wouldn't be able to pick it up.
True sensory deprivation tanks use Epsom salts, not regular salt, in order to provide the correct environment.
Robin lights the wick of her Molotov cocktail before throwing it. Since Vecna is already on fire this would be redundant, the gasoline would ignite from the already existing fire.
In the epic climax of the episode, when Nancy, Robin, and Steve tried to kill Vecna at the loft of the Creel house, he did not try to kill any of them. He just stood and tolerated shots and Molotov cocktails; he didn't use any of his telekinetic powers. But in an earlier episode's (Chapter Seven: The Massacre at Hawkins Lab (2022)) flashback scene, he immediately killed the soldiers who attacked and tried to shoot him.