One of the students scrawls an off-color remark on blackboard in huge letters but when teacher erases it, it is in much smaller letters.
When Chico takes Jane to the drive-in, the license plate number of the stolen Packard changes. When they arrive at the theater it is SV-5875, yet when they start to pull out, the plate has changed to 4N-4290.
When Stanley, Butchy and Wimpy go to steal a car in a lot, it's a dry night. As they are driving the car out of the lot, the roads are wet and the car's windshield is soaked with rain.
After Stanley buys the ring for Frannie, Stanley starts walking with his right arm over Frannie, and his left arm over Annie. In the very next shot, his right arm is over Annie, and his left arm is over Frannie.
The size & positioning of the writing on the chalkboard in the beginning of the movie changes between shots.
The film takes place in 1957 Brooklyn, but the one-way street signs seen are ones that started being used in the late '60s.
The movie is set in 1957 but the motorcycle Chico is riding is a 1966 Harley Davidson FLH.
In the opening credits the camera pans the skyline of NY, and shows the construction of the Twin Towers in the background. The movie is set in the late 1950s and the Twin Towers were not constructed until the 1970s.
Shortly after leaving the jewelry store, some of the characters are standing in front of a shop with a magazine rack. Some of the magazine titles weren't available until the 1960s. The movie is set in the 1950s.
When Stanley is being kicked out of the classroom at the beginning of the film, the camera operator can be seen in the reflection of the classroom door that he is walking towards.