I've personally witnessed two cases where handles were jerked or released from the pilot. One was a case where the plane flew across the circle. These were pre-thong days, but in this case since the plane was low, and the handle was dragged through the grass, it didn't get far before it hit the ground.
The second case was different, and except for extreem good luck, no damage was suffered. The maiden flight on a buddies Sterling Spitfire in So. Cal. At the top of the wing over, he was hit with a jolt of static electricity, so bad that he involuntarily opened hus grip on the handle, an EZ Just. The plane sort of leveled out, making big lazy circles around the swinging handle. He had a full load of fuel on board so it continued to climb until we could no longer see it, just flashes of light reflecting from the wings.
When the engine quit, we watched trying to see where the breeze was taking it. Eventually we spotted it, still making those big circles, and the wind was moving it to the N.E. at about 5-10 MPH.
We didn't have cars, so we jumped on our bikes and took off after it. It went down in a bunch of houses, and we were sure it probably went through some ones large glass window, or worse. hit someone in their back yard.
When we found it, we also found that we had been extreemly lucky. The plane was hanging by the lines, which were drapped over the roof of the house. No one was home, so we disconnected the handle, pulled the plane and the lines free, packed the plane and parts onto our bikes, and left.
It was pure luck no one, or nothing was damaged that day.
Using thongs has, and will prevent accidents.