When you do stockinette, all the knits–the little V’s, are on one side and all the purl bumps are on the other.
What happened here is that you knit a few rows when you should have purled or purled when you should have knit.
When you’re knitting stockinette, look at the knitting itself. If the flat v side is facing you, you need to knit. If the bumpy purls are facing you, you need to purl.
And when you accidentally knit where you meant to purl, and it adds a cute stripe across the mouses back, or an interesting element to the garment you are making, it is no longer a mistake…