Now I know what surgeons go through.
I needed to repair the hem on a pair of trousers. I got out my sewing basket, selected my tools, prepared my materials and mentally rehearsed the procedure one more time. I was now ready to do it live. I accomplished the first part of the procedure by neatly pinning the hem so that it would stay in place while I worked on it. I checked that the vertical crease on the turned up section was exactly in line with the rest of the leg, both front and back. I threaded my needle with just the right length of cotton and penetrated the cloth.
Then I proceeded to sew up the wrong damn leg.