WHen I bring an ambulance in to teach ambulance operations, I always show one thing off on the stretchers. I load some bags on the stretchers, put a little weight on it, then put two people one each end. I show them why it is so important to walk with the stretcher while holding the head and foot ends, and why you CANT wheel it sideways. I put them on a hill, and have them walk sideways, and the stretcher almost always tips.
Then, on flat ground, I have them wheel it sideways again, and simulate what happens when they hit a crack while in load position. By just picking up on the bottom carriage a bit, you can make it tip quite easily.
It really is scary how easily these stretchers tip-- and it only takes one close call before you become more careful.