Stevo's father claimed to have gone to Woodstock. Stevo would have been born in approx. 1963 to have graduated high school in 1981. So his father would have had a six year old son while being a student at Harvard University.
After Stevo yells at the kid with the Union Jack patch, the kid is walking away in the background and his jacket has the Operation Ivy logo on it, a band that was formed in 1987, even though the movie takes place in 1985.
At Mark's house, Stevo is wearing a shirt with the cover of Naked Raygun's "Raygun... Naked Raygun" album on it. Naked Raygun was a band in 1985, but "Raygun... Naked Raygun" was their last album, and would not be released until 1990.
In the flashback at the end of the film (that can be deduced to take place no later than 1976), Steveo and Heroin Bob are ready to play a version of Dungeons & Dragons that didn't exist in 1976. The song playing in the background is "The Trees" by Rush on the 1978 Hemispheres album
When Stevo is introducing people at the party, he introduces Jennifer and says "She's got this brother, his name's Chris. He's this death-rock, punk guy..." and when Chris is introduced later in the film, Stevo hugs him and describes him as a "total hippie".
When Heroin Bob is talking to Stevo while he is laying on the ground with his head resting on a block, Stevo is wearing sunglasses from the camera angle view from the sky. It then focuses on Heroin Bob talking again and then when the focus is back on Stevo, his sunglasses have disappeared.