Check out the video below. It shows what happens after a car, (in this case a Ford Taurus) falls off a two post Car Lift. A two post lift is one type of lift that you must take extra care with. It is important to position the vehicle properly on the lift. Try to keep the center of gravity as close to the columns as possible. If you are going to remove a heavy component, such as a transmission, be careful to place tall jack stands under the vehicle. Any sudden shifts in weight could cause the vehicle to fall off the car lift, as it apparently did in this case. Also be careful to place the lift pads on the proper automobile jacking points, as designated by the manufacturer.
Based on the text in the video, it appears that perhaps the cause in this case was a shift in the weight distribution of the car as parts were removed.
There is always potential for a catastrophic disaster if proper safety procedures are not followed. You can view more about two post lift safety procedures on one of our previous posts.