When Jasper sprays his Loony Whiz, it comes out white, but when Drex crushes the can, it comes out orange.
The internet did exist in 1989 and was used by pioneering computer users. What did not was the modern cell phone system, which is the real reason why Henry would not be able to use his phone in 1989.
The "Where's The Beef?" commercial was popular in 1984 but was long since played out by 1989. It would not have been broadcast in the year depicted.
A later scene depicts Schwoz having a sock on the doorknob of his dorm room. This is a mistake. Male college students only do that when they are engaging in romance with a girl in their room.