Towards the end of the episode Safe House, Stan is sitting down waiting for his burgers.
There is a Hot Stuff crane machine in the background (along with a Centipede). That crane machine is modern.
When Stan is in the fast food place, the Centipede machine is being played by another patron, but is making generic, '80s arcade-style sounds, not the sounds a real Centipede machine.
At 43:00 before Beeman asks "How did he die?", we can see Amador's supposedly dead body make very slight eyelid twitches.
At 13:24, as Philip is leaving Martha's apartment, part of a website address is briefly visible on the awning of the building next door. In 1981, the internet was available only to military and academic institutions and the World Wide Web did not yet exist.
At 18:57, when Beeman has entered Amador's apartment, he switches on a lamp and we see a copy of "Sweetness and Power" by Sidney Mintz. This book was originally published in 1985, four years after this episode takes place.
When Elizabeth talks to Henry about his homework, there are four Reader's Digest Condensed Novels on his bookshelf next to his Hardy Boys books.