Corrected entry: In the final scene, Kid Twist arrives at the betting parlor, whispering to Lonnegan, "Sorry, I couldn't wait." This furthers the plot as Twist can then inform Lonnegan that the wrong horse is going to win. But it would also blow the entire con because Lonnegan would expect Twist to have been downtown at his Western Union office just four minutes earlier when they talked on the phone.
Corrected entry: Just before the sting goes down, the assassin and the bodyguard are shown preparing their weapons. Both place largish suppressers (aka "silencers") on their guns. Both of them are using revolvers. Except for a few, rare models built specifically for the purpose, revolvers can't be effectively suppressed. There is a gap between the cylinder and the barrel that allows some of the expanding gasses and accompanying noise to escape before they can be affected by a suppresser on the end of the barrel. This is one of the most common firearms related mistakes in older and period films.
Corrected entry: The technique used by the 'hit-woman' (Saleno?) is so suspect as to be unbelievable. Remembering that her name is spoken in hushed tones by other hit-men and her professionalism is commented on by the minder employed by Henry Gondorff, one is expecting a first class hit. Instead she simply waits to be chatted up by Robert Redford - not a dead cert by any means, given her plain looks and job as a waitress. When she is disturbed in her plan by another of Lonegan's goons, she follows him into an alleyway and kills him. Firstly, can she be so good at her job that Lonegan does not mind her killing his employees? Secondly, she could quite easily have been seen following him into the alleyway. However she eventually manages to sleep with Redford, pack and leave without getting the urge to shoot him because, as the minder says: "She was a professional. Anyone could have seen you go into her room". Given her less cautious disposal of the other goon and the fact that she had removed all trace of herself from the room by morning you would have thought she could just have shot him on the way out and legged it. Finally she approaches him in an alleyway behind the building to complete the hit. What is she doing there? Redford might have left by the front door three hours later. What if other people had been using the alleyway? Would she have had to embrace him and go off and sleep with him again, or shoot him along with anyone else who was around?