Throughout the fight between Jack and Mr. Jones, the sky changes from overcast to dusk to mid afternoon.
Obvious double in place of Nemo (the knife thrower) doing various stunts.
When the ninja gets impaled by the tree stump, he is holding it upright against his chest.
Wires visible throughout Nemo's knife throwing demonstration.
A great deal of revealing mistakes when Charlie fights the ninjas in the woods. Most notably is the use of wires and dummies throughout the fight scenes. Also, when Charlie first chases after the ninjas, he does a couple of flips in mid air. He is running through the woods, yet the background when he does the flips is of an open sky.
When the mafia boss addresses his ninja students after Tomal's death, it is clearly not the mafia boss himself, but a totally different person. This is evident by the eyes and his lean build. This is possibly footage of an actual teacher and his students.
The patron at the night club won't allow Jack and Charlie into the night club because they are Chinese. However, one of the customers in the fight looks to be Chinese.
After the death of William Chung, Jack says that they are waiting for another master to arrive who can bring men to fight the Japanese. Not only does the new master not ever come, he's never spoken of again.
Nemo should have recognized Jack at the night club. He saw him at the Japanese mob boss's house when they assassinated Master Chung. The other three killers recognized them.
For the most part Jack fights his opponents very honorably. However, in his fight with Nemo in the city park acts very dishonorably by covering his head with a jacket and having Charlie stab him with his own knives.