When Johnathan asks to go with Hopper to check out Lonnie, Hopper gets in his truck and slams the door, causing the mirror to fold in. In the next shot the mirror is extended back out into the normal position.
While it is true that for most of the 20th century AT&T had a monopoly on the telecommunications industry and it's also true that customers could not buy their own phones, they had to rent them from Bell Systems, a telecommunications company owned by AT&T. However the federal government finally broke up the AT&T monopoly on January 8, 1982. After that customers could start buying their own phones. So as this show takes place in November of 1983, after the power surge Joyce would of needed to buy a new phone as Bell Systems no longer rented them out to customers.
When Jonathan listens to The Clash with Will and mentions that the tape contains all good stuff like Bowie and Joy Division, he also mentions The Smiths, whose first album came out in 1984, while this is supposed to be 1983. Not a goof: Jonathan never mentions that he got the song/s from an album. The Smiths had been active since 1982, in May 1983 they released their first single "Hand in Glove", and in October 1983 their second single "This Charming Man". Both were released before the time that the show is set.
A police car at the diner has its emergency lights on. Police cars only put their emergency lights on if there is an emergency; a dead body or a missing persons case is not an emergency.
At 27:11, you can see a Dish Network HD satellite dish mounted on the roof of a house. This particular dish (and its service) did not exist in 1983.
Depeche Mode's "Enjoy the Silence" is on the radio in the background. The series is set in 1983, but the song was not released until 1990.
In a flashback, Jonathan plays The Clash to his younger brother, Will. Afterwards, he mentions several similar music bands including The Smiths. If this flashback happened in early fall of 1983, The Smiths had only released their debut single and were hardly well-known in their native England yet, let alone in a small town in Indiana.
At the beginning of a scene showing Jonathan driving, listening to the Clash on his car stereo, we hear the last few seconds of the song "Go Nowhere" by the band Reagan Youth. The show is set in 1983, but the song was not recorded and released until the following year, 1984.
Though the Star Wars Millennium Falcon toy by Kenner was around in 1983, the version featured was the mid-1990s re-release with updated markings/decals.
When Jonathan Byers takes the 3 flash photographs near the crime scene tape, a loud sound effect suggesting the flash firing is used for each photo. By Steve's pool, when he is taking the covert photos without the flash, the same sound effect is used each time he "clicks" the shutter. There should just be a click.
At 06:53- Joyce shows Hopper the burned telephone handset but she has no matching burns on her face.