At the end of the opening scene, the explosion from the grenade appears to come from an entirely different building altogether.
In the opening scene when The Black Ninja throws a grenade into the building, the criminals scream not only with their mouths closed, but with a stock sound effect.
There are numerous scenes in the film which show characters throwing punches that obviously miss but create a punching sound effect. The most notable of which is the final fight scene between the main villain and the Black Ninja.