Maggie is seen watching old tape of Eddie and the band sing "On the Dark Side", at Frank's college. Yet when we see them at the college later in the film, no camera or film crews are in sight anywhere in the auditorium.
At the hotel concert featuring the imitation "Eddie" band, a male extra is seen both at the bar and standing in front of the stage holding a lighter.
When Maggie is talking to Frank in his classroom, the shoulder strap pad on her purse changes positions from upside down to right side up and moves from her back to her front.
When Eddie ends the song, "Wild Summer Nights", we see him take his guitar off and place it to the side. But in the next shot, showing Eddie from the rear, the guitar is back on, as he waves to the crowd.
When Frank arrives at his home (trailer), you see a copy of the album "Tender Years", about 3/4 of the album is peaking out of the pile. After Doc calls him, he reaches for the album, only 25% of the album is peaking out of the pile.
Early in the movie, we see Frank in his car driving to the high school where he teaches, listening to one of the old Cruisers songs being played on New York City radio station WNEW-FM. Later, when he's reunited with Doc, he says he's teaching in Vineland (NJ). With the range of FM radio limited by "line of sight" (and not bouncing like AM signals do), there is no way he could listen to WNEW-FM that far down in south Jersey.
Wildwood does not have "big motels" or "big hotels" on its boardwalk.
In the beginning of the movie, Maggie tells her colleagues that Rimbaud stopped writing when he was 19 years old. Rimbaud did not stop writing until he was 21 years old.
It's stated that Rimbaud never wrote another word after A season in Hell, 1873 but he wrote and had published two works after that, Illuminations (1874) published in 1886 and Rapport sur l'Ogadine (1883) published in 1884. They also say he stopped writing at age 19, but he wrote until age 29.
In a film showing Wendell being transported out of the motel following his fatal overdose, there is a sign on the motel boasting of "Color TV". Color television sets were not generally in motel rooms until 1966 or 1967.
In the classroom when the 3:00 PM bell rang for the end of the school day, Ridgeway says "I don't wanna see yous for at least 48 hours". If this was on a Friday afternoon, shouldn't he have said 71 or 72 hours or just "til Monday" if this was at the end of a Friday sixth hour class and there isn't any school on weekends. However, this was really just a teacher being sociable with his students as they left class; it wasn't meant to be taken literally or precisely.
When Frank is talking on the boardwalk, several of the passers-by stop to watch the scene.
When the Cruisers play the college, it's supposed to be "Spring Madness" but many of the trees at the college are bare or have brown leaves.
When Eddie and Joanne are shown driving into the Lincoln Tunnel, it is supposed to be 1964, yet a pictographic "steep grade" warning sign (a picture of a truck headed down a triangular shape) is seen mounted on the entrance to the tunnel. Pictographic warning signs were not in use on United States roads in 1964; rather a sign warning "Steep Grade" in text would have been posted.
At the recording studio, when Sal blames Frank for destroying the band ("It was you!"), there's a modern thermostat on the wall over Frank's shoulder.
At the 49:20 mark Frank and JoAnn are walking the college grounds. It is supposed to be Spring 1963. In the background, a 1972 Chevy Impala convertible drives onto the grounds.
When the band is practicing on the roof of Tony Marts, a modern blue and white speed boat can be seen in the water behind them that wouldn't have existed in 1962.
In the concert scene in Tony Mart's, Eddie takes off his guitar after the Cruisers perform "Wild Summer Nights", and the band then goes into "Tender Years". Despite no one onstage playing guitar, the guitar line is still clearly audible throughout the song.
In the scene where Frank (Tom Berenger) is watching Sal (Matthew Laurence) at the nightclub, Sal's band is playing "Betty Lou". Sal takes his hand off of his bass guitar and even slaps it but we still hear bass lines being played.
During the band's musical performances, the sound is nearly always out of sync with the video, especially when they are creating "On The Dark Side" when the tambourine and hand-clapping is 180 degrees out of sync and Frank's hand shadows behind the piano are visible and they are clearly not really playing anything and are barely moving at all.
When Frank is lying in bed, you can see the shadow of the camera moving across his chest as it moves closer.
In the scene where Frank says he is teaching high school in Vineland, he mispronounces Vineland. He says VINE-LAND, when it should be pronounced VINE-lind, as in Cleveland.