Continuity mistake: In the scene when Penny is completely out of it because of her overdose, she and William are dancing. At one point William's left hand touches her face. We see he is wearing a white dotted shirt that comes down to his wrist. When the shot zooms back out, he is wearing a jacket over that shirt and the sleeves are rolled up to his elbows.
Factual error: In a party scene, the song "Burn" by Deep Purple is heard. This tune is sung by David Coverdale on the "Burn" album which was recorded in November 1973 and was published in the beginning of 1974, so it can't have been played in 1973.
Continuity mistake: In the scene toward the end of the movie where William and Anita go home, Elaine comes to meet them on the porch. Will goes inside while Elaine and Anita hug. In the first shot they are hugging cheek to cheek (both faces toward the camera), but the next shot shows them breaking their hug and they are hugging with each chin on the other's shoulder.
Continuity mistake: When William is 11 his eye colour is dark brown. When he is 15 his eye colour blue-green. You would think the makers would have tried to make the two William characters have the same characteristics.
Continuity mistake: In the scene where Penny and William are at the airport and shes ready to go back home after her overdose, her hair is a complete mess, it looks frizzy and dirty. However when she is shown on the plane a few minutes later, her hair is back to normal, curly and clean, like she just did it.
Continuity mistake: In Topeka Kansas, after Russell quits the band and walks away with Ronald Miller the green van that picks him up passes them twice within a second.
Continuity mistake: While the band are talking about what William wrote about them in Rolling Stone and Russell is lying on the bench, the bag on his head has a white label which appears and disappears between shots.
Factual error: In the Cleveland scene when the band is playing you can see the drummer is using a quad beater (the thing that strikes the bass drum) on his drum pedal. These were not available until the 1980s. Similarly the rest of the cymbal stands and hardware is far too modern.
Continuity mistake: In the radio station scene, when Lester Bangs is pulling albums from the shelves and tossing them on the floor, he pulls Yes' Fragile and tosses it. In the very next shot, it's back on the shelf. Two shots later, it's back on the floor - or some other time-warped dimension, but not on the shelf.