You could be making an inadvertent yarn over. Whether you knit in front or back be sure you have the yarn behind the right needle before you insert your needle to knit. Then be sure you bring the yarn to the front between the needle tips so that you can purl next (if you have a purl next) no matter if it is a regular purl or one through the back loop. If you have another knit stitch after a purl be sure to take the needle to the back again between the needle tips before you do your next knit. Failing to move the yarn back and forth between knits and purls will mess things up.
The yarn always needs to be in back to knit and in front to purl.