When Frank and Vito are talking, the level of the foam in Frank's cola changes multiple times. Level with the liquid in the glass, almost to the top of the glass and in between.
In the barber shop when a character is shot, there are two shooters firing at him. Even though both assassins use six-shooter revolvers and never reload, meaning max. 12 shots could be fired, more than 20 shots are heard.
As Gigante is driving to Appalachin, he points out to Genovese a sign for Palmyra. But... Palmyra is a good two hours northwest of Appalachin. No way would they have seen a sign for Palmyra.