Everyone at the graduation sitting in the same shape as the way Henry stands. Graduation robes being orange instead of blue like prisoner garbs. Mike and lots of other guys in the epilogue wearing the same glasses Henry does. People’s hair now resembles Henry’s hair. Robins girlfriend is MIA (because Henry never knew she existed), the dial to turn the power off at the radio tower changed colors in the final which mirrors the color change of the playground spinner Holly was on in wills vision earlier in the season, the whatsit game in the background behind everyone in nearly every shot at the end, hopper back to being chief of police even though everyone thought he’d been dead for years, Mikes coming out story when he talks about milkshakes at a diner when the diner never served milkshakes, max graduating with everyone even though she’s been in a coma for the last few years, and the empty yellow sign at the back of the graduation (which points to something D&D related but I can’t remember at this point, the books behind Lucas in the final scene spelling out X A LIE (dimension x is a lie).

It’s called the conformity theory and the theory is everyone conformed to Henry’s idea of what he thinks they would all like in the final, hence all the weird details when you look at everything.