In a wide shot of the alley shown before Danny talks to the gamblers, a skyscraper with a brightly-lit DTE Energy sign at its pinnacle is visible in the background. DTE Energy didn't exist until 1995, 20 years or so after the scene takes place.
Julius was actually killed from the backseat by Ray Ferritto in 1969 in an LA airport parking garage, several years before its portrayal in the movie. Ferritto used the sound of a jet passing overhead to mask the sound of the gun going off.
When Danny goes to talk to Mike about joining the union, there's a segment where Mike is talking about his kids. You see his son wearing Converse One Star basketball shoes. Converse One Stars weren't debuted until 1974. This scene takes place in 1971.
Many vintage cars are used in this film, most of them mint specimens that are clearly collectors items. But the makers ignored the fact that Cleveland winters usually took a costly toll on vehicles driven year-round. One scene in particular shows Danny's wife leaving him in what appears to be a 1955 Ford. A 20 year old car in Cleveland back in the seventies would have been rusted out dreadfully with holes in the fenders and rocker panels. This beauty looks like it left the showroom last week! Look closely at the cars in the film and you will see they are all in perfect condition even though they would have been 5 or 10 years old.