When Don fights the driver near the and, he has two swords and their scabbards on his belt. After killing him, he runs to the building with the hostages with no scabbards (sayas) and empty hands. He goes down the ladder and has a sword again. Then he confronts the main psycho with both katanas and sayas back in place. He chases them and the swords vanish again.
When Don enters the field to fight the driver, he has no scabbards. Then he pulls a sword from nowhere, and in the next shot there are two scabbards in his belt.
When the Ninja pulls the bullet out of himself and drops it in a bowl, it's obviously a shiny new rifle bullet, not a round fired out of a revolver.
A green light appears as the man opens the case, causing his hand to burn. When the camera is on the man's face, it looks fine, however, when it leaves and returns, it is seen that his entire left face has been burned. It wouldn't take so long for his face to burn like it did during that scene. His face and hand would both burn at the same time.
After the officers finish taking photo's of the dead woman in the garbage can, and move her onto the ground, while still in the garbage can, the actress who is playing that woman is seen repeatedly moving her upper part of her body, even though she is suppose to be dead.
When the ninja finds the woman chained up, he uses his katana to free them, but the chains never break, and the woman just untie themselves. The Ninja doesn't even touch the chains, just swings his katana near them.
When the wife is stabbed at the start of the film, the knife is obviously made of a soft, flexible material, since it's bending as she grasps it.
Even though the headline in the newspaper (The Daily Tribune) changes throughout the movie, the other stories such as "Panic in New York: Menagerie Breaks Loose" never do.
The knives the hoods wield in the fight @30 minutes look like they're made of wood.