I did the flush with stones present, so I was scared too.
It is possible for a stone to get stuck during the cleanse.
This would be painful and require immediate surgery. I passed lots of so called stones, but they latere melted. No doubt nothing but the olive oil/salt mix turning to mineral deposits, but using my net I found three hard stones. It's a ridk you have to think about. I would cetainly get all the info and take your time and do it right.