Zip-lines. They are being installed in private recreation parks and even public parks all over the world. The physics is pretty simple. The cable is essentially a ramp on an decline. Gravity down force helps to translate your potential energy into kinetic energy. Frictional resistance on the cable and some air resistance on your body will limit your speed. Down you go. Fun huh? What happens if the zip-line goes down too, while you are on the line? What happens if the zip-line operator's equipment fails? If they are found to have failed to test and repair then you can sue for your medical bills, right? Wrong! You're screwed. Here is why.
Twenty years ago, around the time I was a graduate student in London Ontario at Western University (formerly called the University of Western Ontario), a clinical pharmacologist (David Bailey) at the Lawson Research Institute in London discovered that chemicals in grapefruit can cause normal doses of several drugs to become overdoses. This results from grapefruit causing the drug to become adsorbed much more efficiently than it would normally.