During Danny's first visit to the Stonewall, a few months prior to the June 1969 riots, the song "Venus" by Shocking Blue is played on the bar's jukebox. The song wasn't released until October of 1969 and wasn't a major hit until 1970, well after the events of the film. Similarly, the song "I'll Take You There" by the Staple Singers, is featured twice on the soundtrack, once as as diegetic music and once as non-diegetic. That song wasn't released until 1972, again well after the events of the film.
In the opening set-up sequence, the police are carrying transparent curved riots shields with reinforced black edges and the word POLICE written across the center of the shield. Standard NYPD riots shields of 1969 were completely flat, did not have reinforced edges, and did not have writing on them (see original photos of the actual riot). The curved polycarbonate shields used for the film were not invented until the 1980s.
During the first scene at the bar, a disco song comes on the jukebox, but disco didn't arrive until 73 or 74. Also, some of the clothes, especially the polyester pants that Trevor wears are more 70's than late 60's. Besides the music errors stated above, the film is rife with goofs.
The timing of Danny's arrival in New York is confused. He leaves Indiana during the fall (it's football season, and the World Series is about to start). But his bus arrives in New York in March (3 months before the Stonewall riots).