It's what's called a spirit line, a deliberate flaw in the pattern to give the weaver or maker a way out, but also a reason to continue making work.
留下一点缺陷。 这被称为 “精神之线” ， 在花样上有意留下缺陷， 不去太苛求纺织工和制陶工人， 同时也是为了 让制作过程得以继续下去。