Un-purled loose yarn on back of stockinette

I have a loose piece of yarn on the back of my stockinette stitch. It is not a dropped stitch, but a piece of yarn that is not purled. Do you know what it is called and how I might fix it?

I can post a picture later, but not now.


A picture would probably give us a better idea, but it sounds like you maybe slipped a stitch instead of working it.