I think it is the cutting of the fabric. Once cut, I can't fix it. It's for the scrap pile. I was cutting out squares the other day and suddenly realized that I was cutting a few of them an inch shorter, until I caught myself. My line of site on my ruler had shifted to the wrong 1/2 inch line. Cursed myself for not using my marking tape on my ruler, since it was a constant repeat of cutting.
As for the rest, if it is the sewing part or quilting part I can always rip it out to fix it..