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.
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.
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.
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.
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.
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.
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.