That can happen even if you're turning the die in by very small amounts. Suppose the little increments you've been using have been working against elastic deformation and then the next increment added on goes into the plastic region. Boing! How did I move the shoulder that much?
That's why I anneal my brass before setting the few FL dies I am using, or my body dies. I measure as I adjust, using a fresh case for each adjustment. Once I find that sweet spot I lock the ring and leave it there.
