I knit conti all the time and occasionally I've found that little uninvited "guest" stitch crashing the party. Now that I think about it, it's usually when I'm working in the round.
What I've found is that in turning the work I've had the working yarn positioned in such a way that it wraps around the left needle. If you don't catch it, it acts like a yarn over and then gets knit into the work and becomes the little gate crasher! When you turn the work, just make sure the working yarn is hanging between the needles.
Hopes this solves the mystery!