Would I fix an oops on a practice quilt? Not unless figuring out how to "fix a mistake" was the reason I was practicing. If it really bugs you, get a white fabric pen & paint that little stitch. My guess is, you'll be the only one who notices it, though. I leave all of my mistakes in as long as the stitches properly formed. If it happened late in my practice session, I will continue practicing that skill until it is mastered before going onto a client quilt.