I actually learned this today when rotating wheels on my Chevy Tahoe. I got into the habit of placing a hockey puck in the cradle of my floor jack to prevent marring. I never had a problem before on my ZHP but, today, when lifting one side of the truck (both front and rear wheels off the ground), the angle of the bottom surface of the frame rail and the forces involved were sufficient to basically "squirt" that hockey puck out along with my floor jack. Fortunately, it wasn't very high and there's plenty of room under there, so there was no damage, but this phenomenon could be very costly and dangerous. Of course, jack stands should be used, but there is a brief period of time between when the car is jacked up and the jack stands are positioned.