I don't know if anyone has realized it, but the show has been breaking the fourth wall and going meta for a long time.
Some of us believe in the Conformity Gate theory and that the finale season is fake. Whether it is completely fake, we're not sure, but the show may have given us some clues, especially through the costumes.
Now that we know how much attention they pay to every detail in the costumes, I think this is intentional, and the Duffer Brothers want us to notice it.
I’m not sure if anyone has mentioned this before.
Wherever this specific pattern appears, the scenes seem to be altered either for us, the audience, or for the characters. My theory is that whenever the pattern is prominently visible, the scenes are completely altered for us by the Mind Flayer and Vecna.
For example, look at everyone's ties in the finale graduation scenes.
The patterns: rings/tunnels, eyes, spirals, etc. Now, I'm not D&D-savvy, but according to my observations, the altered/flayed scenes are indicated by costumes featuring these patterns.
Scenes that may be altered for the audience:
El opening the gate (Mike and Vickie wearing a visible pattern)
Mike and El's scenes after The Sorcerer
Scenes with Max and Vickie
Nancy and Jonathan breaking up
Final graduation scenes (everyone's ties have the pattern)
Scenes in Shock Jock (Mike wearing a visible pattern)
Hopper and Joyce getting engaged
Scenes that may be altered for the characters, especially Mike, Will, Nancy, and Steve wearing the pattern partially visible or camouflaged by jackets):
Starting from Turnbow Trap,
including Mike fighting with the demos
Mike and Will's tower scene
The finale fight scenes with the monster
Mike's scenes with Eleven
My theory is that the scenes after The Sorcerermight be completely fake, a narrative created by the Mind Flayer and shown to both Mike and the audience.
By the beginning of The Sorcerer, Mike appears to be experiencing hallucinations or distorted perceptions (with "F203" mentioned in the background). ICD-10 code F20.3 refers to Undifferentiated Schizophrenia. He starts having visions, and perhaps our minds, as viewers, have been flayed as well.
By The Right Side Up, every scene may be part of that altered reality.
It started from Season 4: Look at Jonathan’s shirt and the background!!! 👀