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.
Continuity mistake: In one of the first scenes with Joe and Curtis reporting to Capt. "wild Bill", Joe and Curtis are just getting ready to walk out of the office. But who's that playing Joe? Certainly not Michael Dudikoff! The next shot Curtis and Joe are comming out of the office and there is Michael Dudikoff again.