When Hopper fixes them his Eggo Extravaganza, Eleven first picks up the waffle with her hands and sprinkles some Mike & Ike candies on it, but when the camera changes she's holding it with a fork and the candies are gone.
Between 13:37-13:47, Hopper draws red circles around the pattern of blue X marks. However, from beginning to end, the red circles are inconsistent between shots as he draws each one.
At 44:32 Joyce Byers' 1976 Ford Pinto is shown with the license plate, 3Xa71D5 prominent. The fictional "Hawkins" is in the fictional "Roane" county. While the Hoosier State license plate is period correct for 1983 issue, Indiana used, at the time, a number format, that used, as the first one or two digits the county number in alphabetical order. The fictional Roane county would be between Ripley (69) and Rush (70). Assuming Roane would supplant Rush and become county 70, the correct format for a Indiana license plate at the time would be 70A1234 (County number, followed by "A", followed by four digits until all possible four digits were consumed then the "A" would be replaced by "B").
When Bob(Sean Astin) and Joyce(Winona Ryder) are enjoying a luncheon outside the store, Sean opens a can of Dr Pepper. In the time this show is set, the can would've had a detachable ring-pull style opener, however Sean opens it and pushes it back as per cans from modern day.
This is actually correct as pull top cans were phased out in the mid 1970's and this is set in 1984.
The water tower near the school has cell phone antennas, which were not in common usage until the late 1990s.
When flipping through the crate of albums in the cabin, Hopper settles on Jim Croce (1972) after bypassing Pink Floyd's "The Dark side of the Moon"(1973) and Supertramp's 1970 debut. The creative team clearly wants to place the collection (and the cabin) squarely in the early 1970's, roughly ten years prior to the events of this episode (1984). However, the edition of "Dark Side" used here is a later release, the Mobile Fidelity Sound Lab version, first released in 1979. Also, there is a price tag on the cover, which clearly shows that the album was purchased at a tag sale, or some other used LP store. While the record shown existed in the world of the film, it would have only been 3 or 4 years old at that point, and would only have been known by audiophiles and super fans. It most certainly would not have shown the wear and tear evidenced here, so it is likely that it was purchased as a prop at a used record store by the creative team without realizing that it was a later re-release.
Two of the four bills in the cash drawer were issued AFTER 1984, based on the US Treasurer signatures: The $1 bears the signature of Rosa Gumataotao Rios, whose signature didn't appear until the 2009 series. The $5 was signed by Catalina Vasquez Villalpando, who appeared on the 1988A series. Of the other two bills, the signature on the $10 is that of Katherine Davalos Ortega, which first appeared on series 1981A and first issued in late 1983. The $20 is a very rare find for the period: Walter O. Woods appeared on series 1928A, 1928B, and 1928C notes issued between 1929-1933, all of which were redeemable in gold, and would be highly unlikely to still be circulating in 1984. This particular note is from the Chicago Federal Reserve District (#7), and is worth several hundred dollars if it is from series 1928C.
At around 26 mins, when the cash drawer opens, only the $1 and $20 bill are correct for the 1980s. The $5 and $10 bills shown are versions the Treasury did not issue until 1993 and 1990, respectively. This can be shown by the curve of text, the location of the Treasurer signature, and the statement about being redeemable for gold on demand (visible on the $20). The $1 bill has stayed the same since 1963, but the $20 was also changed in 1990.
The genus Indirana of Indirana semipalmata tadpole species mentioned by Dustin is described only in 1986. Prior to that it was called Rana semipalmata. Therefore mentioning the genus Indirana would not sync with the time-line of the series plot, since its mostly set in early 1980s.