A good-natured mini-rant, more of a pet peeve, really just a way to show off a rug or two.
I get impatient with rug dealers' penchant for pointing out the 'mistakes' in a handwoven rug, and explaining that there is "always one mistake, because only God is perfect," or some such explanation. This story is told often, and not only for handwoven rugs. I heard a similar note about Japanese ceramics (only the Emperor's bowl is perfect.) And I don't doubt that some people, somewhere, at some time, have practiced adding a deliberate anomaly to their work, as a sign of respect for some entity, or as a demonstration of humility, or to avoid the evil eye. That's perfectly plausible. But this endless game of finding "the mistake" in a rug, which is repeated by the owners of the rug at every opportunity, is tiresome because it misses the point. And the point is that the rug is handmade, of course it's not perfect!
The rugs I own have not one, but many delightful variations of shape here and there, and I see them as signs of the humans who wove them, making choices as necessary and weaving along without too much concern for perfection. I don't believe that they considered picking it out and making it 'perfect', but decided not to because of some traditional practice. I believe they didn't much care, as long as the whole thing worked and looked harmonious.
This is most obvious in a soumak rug from Dagestan, purchased from pilgrims in Damascus. The design has concentric rectangular borders, building out from a central grid. Concentric borders, as anyone who has made a quilt top knows, present the challenge of numerous corners. None of the repeated patterns in the Dagestani rug borders are perfectly aligned with their length, making for some entertaining improvisation in each of the corners. This rug, more than any other I've seen, highlights the tendency to make it work. Because the motifs are not simple, and they do work within a grid, but clearly counting and planning it all out ahead of time to fit just right was not part of the agenda. Someone familiar with the tradition might even be able to tell in which direction the rug was woven. Certain motifs are more condensed on one end and spread apart on the other, and I wonder whether the condensed end is the beginning (which would be my guess, knowing from my own experience how the enthusiasm for intricacy can fade later in a project.)
The variations on motif shapes and placement are a view into the mind of the weaver. They not only attest to the handworked nature of the rug, but show the thinking, responsive person who put him or herself into the piece. If something is perfectly executed, we can perhaps admire the skill, but we don't connect with the humanity of the craftsperson quite so easily as with a more improvised work.