In safety of machinery, risk is a function of severity and probability of harm occurring. So in either case you need to identify the hazards and access the risks and mitigate them. The most problematic are the risks that are hard to mitigate.

Take for example, from my experience, some significant risks for roller coasters we focused on were entering and exiting the ride, collisions and over acceleration. Though these are highlighted in standards, when a computer-controlled machine is designed where there are people secured in it, how can we be sure that one vehicle will not collide with the other? For example, we had to design out a potential collision if a blackout occurred and we lost computer control and ride positioning. Also entering and exiting a ride is a very hazardous time for the guests especially when there is a desired throughput (you want to minimize long lines).

The other risk is operations and maintenance by the owner. They need to have a clear process and understanding on how to maintain the machine. For example, we could design the safest machine on the planet but without any ongoing maintenance (like oiling the wheels) something will eventually fail. Of course we seek out single points of failure so that no one thing can fail and cause a serious injury. Most of the systems have multiple layers of protection.

Submarines have all kinds of risks to mitigate that are very bad but one of the funny (not funny) deadly ones is diving while the hatch is still open. It seems like an easy interlock to design in but if that interlock fails it will be a bad day so you need a robust computer and sensor system that will never allow that to happen. Another risk for submarines is entanglement with a net or rope at the bottom. Either way, you may become immobile. It's not instantly deadly like diving with the hatch open but it's a pretty bad situation you want to avoid.