When the group arrives at Suzie's house, the little boy answers the door and immediately shoots Mike in the forehead with a yellow suction cup arrow. You can see in the boy's quiver that there are several different colored arrows, but no other yellow ones. After Mike removes the arrow from his forehead, the camera immediately shifts back to the boy who has the yellow arrow set in the bow ready to fire again.
When Eleven is practicing with the disks falling down in the game room the spot at the bottom for number 8 is empty. However in the next shot where the board is seen from a distance there is a disk in the spot for number 8.
When Jason is addressing the citizens of Hawkins after interrupting police officer Daniels, the collar of his polo shirt changes between shots.
Disregarding the fact that IP geolocation doesn't exist in 1986, there wouldn't be an IP address to trace. They dialed directly, and did not use the internet at all. The peer-to-peer connection did not use TCP/IP, the technology that informs IP addresses.
Within this instance and several episodes, very large walkie-talkies are provided by Gaten's character and used. These devices wouldn't be able to communicate so clearly and at all over such great implied distances. Also, if the device stayed on to accept incoming speaking, it would've needed the antenna up plus be in a squelch / discerning or staticky mode, and battery drain would be frequent, like every 2 hours.
Suzy says that MIke and the others drove 3,000 miles from Los Angeles to Salt Lake City. It's only 689 miles. It's approximately 3,000 miles to drive from one end of the country to the other.
Any lake in central Indiana at the end of winter, where the daytime temperatures are usually in the mid-50's and the nights often freezing, would be very cold. When Steve and the others dive into Lover's Lake, they show no discomfort with the cold water temperature; Steve isn't even in a hurry to get out of the icy water after his dive to the bottom (which would be even colder), leading to his capture.
Eddie (Joseph Quinn) says his Casio LCD watch was damaged underwater, but no water damage is seen: no foggy glass, no condensation, no LCD regions improperly illuminated or darkened. Instead the time is quite clearly frozen in place, as if it were on a water-damaged analog watch.
When Suzie connects through the modem with the computer, HTML code is shown, which includes CSS and an "H T T P S" link. The show takes place in 1986, but both CSS and "H T T P S" were not introduced until 1994.
IP/Internet Geolocation technology initial development started in 1999. And first patents were granted in 2004.
Would make it impossible to use in 1986.
When Suzie is hacking into Nina (at around 54 minutes) several items appear in the scrolling "code" that have not yet been invented. HTML wasn't introduced until 1993. CSS wasn't invented until 1996. The BMP and PNG format wasn't introduced until 1995.
On the desktop of the computer used by Suzie (an Amiga 1000) there is an icon of the floppy of the Operating System Workbench 1.3 that will come out only in 1988.
While Suzie uses her father's computer to find the Nina Project location via I.P. address, the coding language CSS (Cascading Style Sheets) is visible on the computer screen. CSS was not invented until the 1990's. More specifically CSS Flexbox was not invented until the 2000's.
At around 43:36, Dr Brenner's voice is heard but his mouth isn't moving.