When Hooker first meets Gondorff he says "he didn't tell me you were a screw-up." However, when he says 'screw-up' you can't distinctly see his lips form the letter F, meaning that he actually said another, ruder world that was covered up later. [This was not on the DVD version, presumably only the censored TV version.] Corrected by tw_stuart

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. [The key word is "effectively", from the submitters description it appears that the silencers offer some noise reduction which is presumably enough for their needs.] Corrected by tw_stuart

In the scene where Doyle Lonnegan is set up to be too late to make his first bet, J.J. Singleton, who is calling the race over the loudspeaker, announces the eventual winner's odds at 3-1. After the race, Singleton says the horse paid $6 to win. A horse at 3-1 odds pays $8 to win. [In fact he says the horse paid $6.00 FOR the win, not TO win and that is perfectly accurate. Haven't you ever played the ponies? A 3 to 1 winner pays $6.00 for the win on a $2.00 bet and you get your $2.00 back, so in all it pays $8.00. The race caller was right on the money.]

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? [First, it's apparent Lonnegan doesn't mind Selino knocking off one of his other hitman. In one of the scenes in his Chicago office, he's talking with his clerk about Riley and Cole getting "bounced off a job." When the clerk tells him that one of the duo - I forget which - has stuck behind to try to finish the job, Lonnegan tells him "he's breaking the rules and Selino won't like it," expressing his apathy towards the hitman. Secondly, when Hooker goes into Selino's room, the landlady across the hall opens her door and sees Hooker. That is why Selino doesn't kill him then. When she kills Riley or Cole (whichever), it was in an alley at 2 or 3 in the morning. May be a stretch, but there's less of a chance someone else is going to be around. I think she knew what she was doing.]