When Charlie and Bailey are on the roof, his hand position (wrapped up in her hair) changes with the camera angle. From behind Bailey, his fingers are straight (1:35:42 & 1:35:52). From behind Charlie, his fingers are curled (1:35:47 & 1:35:54).
At the beginning of the fight between Noisy Boy and Midas as Charlie and Max are standing on the rising platform on the edge of the ring, Max first has his arms down by his sides (28:16 to 28:21 from behind), then he is holding a briefcase under his right arm with his left hand steadying its front (28:22 to 28:26 from in front), then both arms are at his sides and his right hand is holding the briefcase by its handle (28:27 to 28:29 from behind) when Charlie hands him a second bag so he is holding one bag in each hand as he steps into the ring.
Given that Hugh Jackman is left-handed, Atom is seen at times mirroring Charlie rather than mimicking him. In many scenes, especially in the fight between Atom and Zeus, Atom is clearly fighting right-handed while Charlie is shadow boxing left-handed. This is perhaps the reason why Atom is seen to be alternating between mimicking and mirroring even though according to how shadow boxing is explained in the film he should be only mimicking. This is also easily seen, though, when you notice the person running the shadow function either facing Atom or not. The shadow apparently mirrors when the operator is facing him and in mimic when not.
Throughout the film, Atom's shadow function switches back and forth between mirror and mimic for no apparent reason or command from Charlie or Max. It appears to have no difference except that it is apparent the shadow function goes mirror as the robot and the person are face to face and mimic when the robot is observing the human who's not looking at him.