You see that stitch marker down near the bottom, far from the knitting needles? That's holding a dropped stitch. I discovered this dropped stitch when it came time to attach the sleeve to the sweater body. *After* I cut the yarn, of course.
1. Secure the stitch and leave the sleeve as is.
2. Reknit the sleeve.
I think I'm leaning towards the second option. However, I won't make the decision tonight. (I may even start a third sleeve with fresh yarn, just to be on the safe side. I bought plenty of yarn, since I wasn't sure if I'd knit this size, or one size larger.)