This sounds like what happens to me often, I'm a bit clumsy or unlucky or tinking and my last stitch of a completed row falls off the needle and undoes itself, and I put the (now unknitted) last stitch back on the needle, so the working wool is coming off the second-to-last stitch.
To fix it I put the full needle in my right hand and the empty needle in my left. Now I put the last stitch onto the empty left needle. I have (for example) nineteen knitted stitches on the right needle with the wool coming off that last stitch, and one unknitted stitch on the left needle, just like you do at the end of a row when you haven't knitted that one last stitch left. Knit this normally, just as you would do at the end of the row.