After unlocking the doors, Bob can be seen through the security cams system grabbing the gun before leaving the room. The desks remain empty except for the monitors and keyboards. Then, before he returns to set on the sprinklers, the gun can be seen on the desk through the cameras, but when Bob returns to the computer, there's no gun in the desk. The second time he leaves the room, he forgets the gun, which is lying on the same spot where it was shown on the security footage.
BASIC was a real programming language which was highly popular back in the 1980s. But the acronym stands for "Beginner's All-purpose Symbolic Instruction Code," the important word here being "beginner." It was a simple, human-readable programming language that even complete novices could master quickly. No programmer would ever have considered writing security or any other mission-critical code, least of all for a secret government scientific laboratory, in BASIC. They'd have used COBOL or FORTRAN or even PASCAL.
Joyce reminds Will about his 120-count box of crayons. The 120 Crayon box by Crayola was not introduced until 1998. There is also no historical record of another crayon brand producing a crayon box with 120 colors.
At 14:56- the breakers are in the "off" position. If they had "tripped'' the levers would be midway between the "off" and "on" positions.
When Nancy announces that the lights had come back on within the government facility, all of the lights seem to be working perfectly. At least some of them should be flickering, since the demogorgon and demodogs have some type of effect on them.
When Jonathan and Nancy arrive at the guard house, there are no guards there. The guards wouldn't simply abandon their post when the alarm sounded. Also, when the power went out and didn't allow the gate to open, the guards would have been trapped outside with Jonathan and Nancy.
As Joyce and Jim narrowly escape the demadogs in the Hawkins Lab, they exit through a set of doors that have no locking mechanism and push out to exit. Seconds later, five or six demadogs slam against the same set of doors but cannot push them open.