Before attempting to find Billy, Max turns on all the showers in the locker room for El. After El's vision, the showers can be seen behind her and all of them are off.
The LP being played is not a Don McLean record label, it is neither the original from 1971 on A&R nor the 1980 re-release on Liberty. And American Pie on both versions is the first song, not near the end as is shown on the record player.
The license plate on Billy's circa '70s car has three letters and then three numbers. The blue California license plates of this era had three numbers and then three letters. It's possible that this could be a vanity plate, but vanity plates are not allowed to resemble a standard issue plate.
Eleven's Coke bottle doesn't look like the ones that were sold in the 1980s. It should be longer and had red paint.