I knit one present this year.  A pair of those mitts.  And when I finished I laid them on top of each other and low and behold they were two different lengths.  I was off by two rows.  I stretched and pulled and thought, heck, they are fine.  But then the voice in my head said, really?!  So... I cast on for another mitt.
And my awesome hairstylist will get a pair and an extra mitt.  A bonus mitt let's say.  In case she looses one. 😊