Makin this one STICKY, for our new-to-knitting friends!
My most frequent reason for making this mistake when I was new was bringing the yarn up and over the needle after the last stitch in the row. Go get your knitting & see how this happens.....bring your working yarn up and over the needle....do you see how that causes the "legs" of the last stitch on the needle to look like TWO stitches? Now, bring the yarn over the front toward you. Now it's one stitch again, right? That's how it SHOULD look.