Something that happens to me sometimes is that I'll pick up a loop from the row below the current one along with the stitch I'm actually trying to knit. That makes two loops on the needle, so when I get back to it it looks like 2 stitches. (And if you knit them both, it WILL be 2 stitches thereafter.) If that's what you're doing it should be easy to tell because the stitches will get excruciatingly tight if you're doing a lot of it, especially if it happens a lot in the same row.
As someone else pointed out, the working yarn can get into the act too and disguise itself as a stitch (and then actually become one later on). The working yarn will wrap around the right hand needle (an accidental "yarn over") and when you get back to it... you've actually made an increase without knowing it. These are pretty easy to spot too, because a YO increase leaves a hole in the fabric. (It also looks different when you knit it, but it's hard to describe that in text.)
It probably isn't as common, but you might also be knitting the same stitch twice. That can happen if you're fighting the yarn anyway and lose track of what you just did. If you're having a lot of trouble pulling the yarn through the stitch with the RH needle, it might be easy to forget to take the stitch you just finished off the LH needle... and knit it a second time for good measure. this will get tighter than usual too, and look odd when you DO take it off, but (again) it's hard to describe.
Most likely it's a combination of these and possibly other things that are causing your mystery stitches. And eventually you'll be doing all of them on purpose in more advanced patterns.
Judging from the picture, I'd say you're doing pretty well for one day's experience! I can tell you for sure I didn't have nearly so much to show for my first day.
Just keep calm and carry yarn.