Dropped stitches are one of those knitting problems that can happen to anyone at any time. No matter your skill level, if you're not paying close enough attention to your knitting or knitting more quickly than you should be, it's very possible to slip an extra stitch off the needle as you're buzzing along, resulting in a dropped stitch.
I've found dropped stitches many, many inches later in a project, at which point they're pretty hard to fix in the traditional way (if the stitch doesn't matter for the pattern I'm working and its loss isn't very noticable, I've been known to take a new piece of yarn and just tack the lost loop down so it won't unravel further). But if you catch it within a couple of rows of the drop, it's really quite easy to pick up a dropped stitch, especially in Stockinette.
It's easiest to pick up stitches on the knit side, so you can always flip your work over if you discover the drop while you're on the purl side. And if you're picking up a stitch in a pattern stitch you need to pay careful attention to whether the stitch needs to be a knit or a purl on that particular row, which can be a little tricky. You may find that you'll be happier with your finished results if you just rip back to the dropped stitch and continue on form there. Fixing mistakes is all about what you the knitter are comfortable with, because the vast majority of the world will never notice your mistakes!