Mishnah Berurah — משנה ברורה

For that is not called "a corner" - And this prevents [one from fulfilling the mitzvah], even after the fact. And even if [he made the hole too high and then] afterwards he made the ציצית (ie. tied the knots and windings), and even if he made just one set of windings, he [realized and then] cut the hole [downward] so that the strings hang lower [in the proper area of the corner], even in such a case it is invalid because of "You shall make, and not from that which is [already] made". And the same applies in the opposite [case], where he affixed the ציצית [in the hole when the hole was] within [the length of] an "agudal" (i.e from the knuckle of the thumb until the end of its fingernail) [from the edge of the garment], and [after he realized] he cut upwards into the garment and pulled up the strings [so that they are now] higher than an "agudal" [from the edge] and then sewed below that point or affixed a patch on the corner [to hold it in place], this is also "You shall make, and not from that which is [already] made". A wool טלית which had ציצית affixed to it according to halacha, and then they found among the tassels of silk that were made on the edge of the טלית (ie. they would do this for adornment), threads of linen (which constitutes shatnez - a forbidden mixture of fabrics), and they had to remove the silk tassels from the טלית... [in such a case] it is proper to be stringent and untie the ציצית strings and then go back and tie them.