I read it probably 25 years ago, most likely in a Norton anthology.
The narrator was a middle-eastern rugmaker of great skill, nearing the end of his life. The custom was to deliberately introduce a flaw into every work, as only Allah can create something perfect. He talks about how some rugmakers make a point of making the flaw as visible as possible, thereby bragging about their faith and humility; others minimized it. The poem ended with him determined to make one last, great rug, the culmination of his lifetime and his skills, with no flaw at all.
Google has failed me, so I turn to the Dope. Dope?