I don't know if anyone has experienced this but, here goes.  A couple years
ago I had a flat and was on a slight incline.  With the car in park and the
parking brake set, it rolled back off the jack.  A month ago, my wife called
and said she had a flat.  The car again was on a slight incline.  Again,
with the car in park and the brake set, I just got the lug nuts on a turn or
two, and the car rolled backward and came off the jack.  This seems like a
serious saftey issue. I've never experiened this with any other car I've
owned , nor heard of it happening.
Any comments from the Lexus group?