Revealing mistake: During the final sword-fight, in at least two shots, when the camera is facing Chon Wang and Rathbone's back is towards camera, you can tell that Rathbone is being played by an Asian stunt-person for certain moments. His hair doesn't match perfectly, his body type is SLIGHTLY different and you can even occasionally catch very brief glimpses of the stuntman's face in shots where he twirls around. (Most noticeable early on in the fight during the stunt where Chon jumps up onto the railings after blocking and dodging several swipes from Rathbone).
Revealing mistake: During the "Singing in the Rain" segment of the big fight after Chon and Roy arrive in London, watch carefully when Chon "lifts" himself with one arm, raising his legs to the side to dodge a strike. It's clear that wires were used, as his body somewhat unnaturally lifts up.