I had this happen to me a few weeks ago. I was replacing the changing the airbags to coils on my wifes GX470, I had two jack stand under the frame, a third set off to the side and the jack under the receiver hitch. The wheels were chocked, I thought I was safe. Some how, the whole car went sideways landing on top of me. Thank goodness I left the tires on or I wouldnt be here. The wife came out a couple minutes after it happened while I was still under the car. Talk about a helpless feeling. I was able to inch my way out from under it. I used cribbing to finish the job. The tird jack stand caught the running board, and I think that is what helped keep it from crushing me. It still gave me a heck of a chest compression. Not only have jack stands, but if possible, have someone there with you, NOT under the vehicle.