Proper fitting tack is as important to a horseman, as a navigator is to a pilot.

Mix and matching/rehasing any part of a horse's tack wrong is like wearing clothes that are too tight or too loose. It just doesn't feel right, and you are uncomfortable. We as humans have a spoken language and we can express our discomfort. A horse, or any animal for at matter can only express their discomfort by rebelling or fighting. Often leading the animal to be corrected for the wrong reason, by unskilled individuals.
I made a comment the other day in regards to ill fitting/rehashed tack on a European liberty act. Least we think it is a "Nation" thing, look at the nose band and cheek strap on this American circus horse. Unless he is "stretching" the cheek strap that bad with the over check. If that is the case, he has some real issues.