Goofs

Right after Eddie gives Steve his jean vest to wear, they are thrown to the ground by an earthquake. When the camera cuts to Eddie, he is still wearing the jean vest, then in the next shot, it's gone again.

A chess game is played between two children in the rainbow room during El's flashback. The flashback serves as the same intro from Chapter One: The Hellfire Club (2022). However, the chess position has changed considerably. This is easily identified by counting the number of pieces remaining on the board.

Presuming all the children were numbered as they joined the group, then Two should have known who One really was.

Considering the age difference between One and Two, Dr. Brenner likely transitioned One into his role as orderly before he was given a chance to meet Two.

The discovery by Nancy that the Upside-Down version of Hawkins is a (dark, decaying, lifeless) reflection of what it was on November 6, 1983, makes it impossible that Will, trapped in the Upside-Down, would have been able to see the lights that Joyce had added to their house and the alphabet she had painted on the wall to use them to communicate with her.

Nancy and the others in the Upside-Down are able to perceive where lights are and manipulate them, including the Lite-Brite that was moved, even though they cannot necessarily see the object itself.

The gang communicates from the upside-down by making lights turn off and on. The Lite-Brite they manipulate only has one light bulb. The patterns are made with colored plastic pegs. If they were turning the single bulb off and on, the entire set of pegs would glow off and on, not individual ones to create letters.

In both scenes showing a chessboard, the chess pieces are incorrectly set up. The kings and queens are swapped. For the chess set used, the peacock is the queen and the eagle is the king.

In order to privately pass secret knowledge to Eleven, an orderly takes her aside to speak under the guise of a game of chess. Both players make useless openings and moves, and surely are both intelligent enough to play a much more adaptive and skillful game - it would have been noticed by the cameras and the operators behind them.