The 1903 The Oriental Rug: A Monograph on Eastern Rugs and Carpets says:
Deliberately, if necessary, it must show some defect, in proof that Allah alone is perfect.
The December 1903 American Carpet and Upholstery Journal, Volume 21 says:
A leading tenet in the Oriental faith is that Allah only is perfect, and that man should not attempt to perfect anything.
The 1901 Oriental Rug Weaving says:
Again you will find the designs of rugs slightly irregular or imperfect. This is not the result of carelessness or accident, but is done intentionally by some devout artist. It is thus intended to show that nothing but Allah (God) can be perfect.
The 1911 The practical book of oriental rugs says:
The Turks, being orthodox Mohammedans, never weave figures of animals, birds or human beings into their rugs, as the teachings of the Koran forbid it lest it should lead to idolatry. Neither do they, as a rule, make their rugs symmetrical, their idea being to symbolize the fact that only Allah is perfect. The Persians and Chinese, however, being more liberal, exercise greater freedom in these respects, and in some of their old hunting rugs, of which but few remain, are depicted animals of all kinds.