Other mistake: When Joe (The American Ninja) pushes the girl out of the moving truck, one sees the stuntMAN (playing a girl) mis-time the fall and land on a large clump of rock and dirt, after which, before the scene cuts, we see the stuntman curl over in pain. Obviously a very painful fall for the stuntman and a very very funny scene for the viewers.
Other mistake: At the end of all the beach fighting scenes Joe (the American Ninja) and his side-kick jump off a rather large cliff into a waiting boat. Watch how they land. Is it a Ninja landing technique or the fact that they jump and land, with such ease, from an obviously lower platform. (Note also the very bad acting involved in trying to conceal this fact!)
Other mistake: When Joe escapes with the girl and runs out the back of the bar with her, he fights one particular man who confronts him in the alley. In the fight scene, Joe flips him over his head with both his hands. But if you look closely, you will notice that Joe's hands aren't even touching the guy. His wrists are just pressed against the back of his attacker who then pretends to fall down.