The first police wagon picks up the rioters/looters and leaves. A second later, the same wagon, with the same license plate, pulls back up empty to reload.
The blood over Julie's eye changes slightly from one scene to the next. Most of the time, she has the same blood smear, but in one scene, the blood is on her eyelid when it isn't in the rest of the scenes.
During the rioting there is a burnt out Volkswagen Beetle smoldering. The rear burnt out tail lamps are clearly the larger 'elephant foot' style not introduced till 1973.
The film is set in 1967. The Dramatics sing a line from 'What You See is What You Get' which came out in 1971.
In one scene, a marked Michigan State Police vehicle leaves the scene with a single overhead blue beacon illuminated. Michigan State Police vehicles have always had a single red beacon.
On the chalkboard map in the police station, Livernois Ave. is misspelled Livenois.
The telephones in the hotel rooms and elsewhere have handsets with modular connectors and flexible cords. Phones like that weren't available nationwide until the 1970s, but they were available in Detroit in 1961.
A grounded outlet is in one of the interrogation rooms. Outlets generally weren't grounded in 1967. However, the National Electrical Code first mentioned grounding receptacles in 1959. The 1968 code required grounded receptacles in new work.
At about the 14:00 mark, a police officer beats a protester standing next to a wall with a night stick. The night stick flexes as the officer swings, indicating that it's made out of rubber.
Most of the police officers' haircuts are against regulations for the time.
A shot in the first half of the movie includes the back of a parked Volkswagen Beetle, which fills most of the frame for a few seconds. The car's rear deck lid has four large vertical cooling vent slots, a design introduced in 1972.
When protesters run down 12th Street, mobile phone panels and microwave dishes are visible.
In one scene, a wall light has a compact fluorescent bulb. They were invented in the 1990s, and weren't widely used until the 2000s.
In the film, The Algiers Motel is across a wooded city block, on a parallel street to the Great Lakes Mutual Insurance building being guarded by police and the National Guard. The troops and police can see the motel and the windows of the Annex house straight on, because the motel faces the front of that building. In real life, the Great Lakes building is right next door to the Algiers, on the same side of the same street. People in front of the building wouldn't have been able to see the front of the motel or the Annex house, at least not at the clear angle depicted in the film. No one in the Annex house would've been able to see the front of the Great Lakes building.
The film has several people aware of a starting pistol that sounds very real. This included the two white women, who have not met any of the others before now. But not one person thinks to mention it to the police when they keep demanding the gun.