In the bar scene with Jackie and Mickey, the drinks on the table seem to be constantly changing. The levels in the glasses change, the number of glasses on the table change and the arrangement of the glasses change.
Just before his beat-down in the rain by Steve and Barry, Markie is sitting in the back seat of the car, completely dry. In the very next scene, having just stepped out of the car, his hair is plastered to his head and his shirt is soaking wet.
At the beginning of the movie, Russell and Frankie are discussing the possibility of doing or not doing the coup. Russell is eating an ice cream. Suddenly, in a next shot, the ice cream has disappeared.
When the brothers Caprio drive upon the road and park at the right side of the road in which Markie Trattmans house is set, a white luxury car is seen on the left side of the road. After a dialogue between the Carpio brothers a next shot of the street is shown when Markie Trattman drives down the road to park at the left side, this white luxury car disappears.
The sawn-off shotgun Russell has in the robbery scene would fail to work correctly, as there is no barrel. If fired there would be no compression leading to the powder charge not burning correctly. It wouldn't be effective as a weapon.
The goof items below may give away important plot points.
Incorrectly regarded as goofs
When John Amato drives up to the lady's apartment to drop her off, he gets out on the right hand side of the Cadillac. When he returns to the car to drive away, he gets in on the left hand side where he is shot by Jackie. (The film takes place in the US and the car is left hand drive. She drove and he was a passenger. When he was leaving, he approached the left side of the car because he was now the driver.)