There is deep Shaker heritage here in this part of New Hampshire and in my family. While I cannot speak for the Shaker or Amish quilt makers, the Persian rug makers, the Navajo blanket makers or indeed countless other artisans around the world about whom this idea has been suggested, I don't much care who you heard it from, it's a myth.
None of those groups, or any with the deep faith in a supreme being that they all share, are arrogant enough to believe that they have to put a mistake in their work INTENTIONALLY for it to be imperfect. To feel you have to put a purposeful flaw in your work, would be the very height of pride...something totally absent in Shaker life. You would have to think your work was perfect before the addition of the mistake.
My mother worked in the archives at Canterbury Shaker Village for years and never ran across any verification of intentional flaws in any of their work. It is so opposed to all of their beliefs that it is absurd. I stand with Loretta, it's a myth, pure and simple.