Exercise balls are dangerous

I bought 1 last week. My doc said that it might help my back & posture. Well my hubby was on it earlier & saying he could balance on it with his feet up in the air. He was in the kitchen adding child locks to the drawer. So I decided to sit in there with him on the ball on the kitchen floor. I decided to stretch & lean back. Well the ball started rolling & I fell backwards. I knocked Tara over & hit the stove door really hard with the back of my head, bruised my elbow & stratched my foot. My head still hurts & my neck. I knew better than to sit on the floor with it, but did it anyway.