I do dumb things all the time. Usually it's on the order of flippy corners sewn on the wrong diagonal, and are noticed before it gets too far along. I just ripped out a beak on an owl block. The beak was supposed to be green but I made it light gray, the same color of the face - all 4 flippy corners the wrong color. Didn't notice it until the whole block was sewn together. It's fixed now.