Factual error: Adam is excited the new Atari video game of "E.T.: The Extraterrestrial" was just released. In the same episode, Uncle Marvin is telling Barry he just saw the movie "Wall Street." Problem is, the "E.T." Video game was released in December, 1982, while "Wall Street" was released in December, 1987.
Factual error: At the very beginning of the show, they are standing in line to watch the movie ET in "1980 something." The kid starts asking if Yoda was just a puppet and if ALF was just a puppet. ET came out in 1982. ALF did not come out until 1986. Later in the show, they mention Kokomo by The Beach Boys which did not come out until 1988. ET was out of cinemas by then.
Continuity mistake: After Adam crosses the railroad tracks in Grandpa's car, the next quick view through the front of the windshield to the back window of their car shows a street scene, and not the railroad crossing lights, gates, or tracks that should be there.