As a secretary I would often not see a mistake in a letter or document I had typed, despite reading it through several times, it was only when the boss pointed it out, and sometimes he didn't see it either, I would spot it in the copy after I had posted the letter. I have done the same in a quilt. I noticed whilst I was hand quilting my son's wedding quilt that one of the 4" square was back to front. I had to put an applique over it. It is like looking in the cupboard and not seeing something that is staring you in the face. There must be a psychological answer to this strange phenomenon.