In the scene where Petey and Dewey lock themselves in the DJ booth, Greene puts on an LP featuring Les McCann and Eddie Harris' song "Compared to What". The album the song appears on, 'Swiss Movement', wasn't released until 1969, though the scene predates Martin Luther King's assassination in April 1968. Another circa-1968 montage, set to Sly & the Family Stone's "I Want to Take You Higher", is set to a song released in 1969.
In the opening scene, we see Petey Greene thumbing through a selection of vinyl records, eventually picking out the album 'It's A New Day - Let A Man Come In'. At this point in the film, it would appear that Petey is still in prison, however the James Brown album that he picks out was not released until June 1970. Moreover, the track played - 'It's A Man's Man's Man's World' - is a different version to that which appears on this 1970 album. Indeed, the version that is heard at the beginning of the film was not available commercially until the release of the 'Star Time' box set in 1991.
A DJ plays "I'm Gonna Make You Love Me" by Diana Ross & The Supremes and The Temptations, a record which was released seven months after the assassination of Martin Luther King, Jr. (which takes place later on in the film).
The segment that features the James Brown concert, after the assassination of ML King, opens with the song 'Say It Loud' (I'm Black and I'm Proud). This song was never part of the set list of the original concert because the song wasn't written and recorded until August 1968 - 4 months after the assassination.
In a scene set in 1966, Petey Greene repeatedly calls Dewey Hughes "Mr. Tibbs" as a disparaging comparison to Sidney Poitier's buttoned-down character in the film In the Heat of the Night; however, that film was not released until August 1967. However, the reference could be to the novel by John Ball, which was released in 1965. In another scene set in the same period, Greene makes a reference to Muhammad Ali being stripped of his boxing title due to refusing induction into the army; however, Ali's title was not revoked until April 1967.
James Brown played a hugely important concert the day after Dr. King's assassination. It was broadcast live on local TV to help keep people off the streets. But the concert, and the broadcast, were both in Boston. Why Petey Greene would have been a part of that is mystifying.
In the scene where Dewey overhears 3 men at a bar talking about Petey's show, there is a road sign mounted on the wall behind the men. The writing on the sign (indicating the way to Teaneck) is backwards.