Other mistake: In Gene Kelly's 'Singing in the Rain' sequence, the taps he makes do not match up with the actual movements of his feet - they are much more complicated than the steps he makes. Since one can't tap dance in wet loafers, Kelly had to do the dance in the studio, and the taps were added into the picture. He apparently got a little overzealous.
Other mistake: During the car chase an Indian shoots an arrow that bounces off the car's license plate and comes back and hits the Indian. But a second before the arrow hits the Indian you can see him sitting on his horse with his eyes closed and a look of pain on his face as though the arrow had already hit him.
Other mistake: Before the trial by combat, when Richard Taylor and George Sanders choose their weapons, they are brought to them on their shields. But this happens to quickly, right after the choices were announced, as if predetermined.
Other mistake: Maureen O'Hara's brother punches John Wayne. You can tell how far away from him Wayne is, and the sound of the punch is a touch off. He's punching air.