I always have to fix it. That is, if I notice it while working on it. Sometimes, in my new projects, I did not notice until I was done, like a twisted stitch or whatever. My very first project I had major errors on and did not fix them.... well, I thought they were major, my sister did not notice.... anyway, everytime I see that scarf I get irritated inside. So, now, I have to undo or frog my mistake. Now, I need to learn how to unknit so I don't just rrrrriiiiippppp and then thread the needle back through the loops because that is a major PITA!