The best designs are ones which are intrinsically safe through their simple design rather than requiring complexity and a patch work of additional systems to manage the inherent flaws of the core system.
An easy way to visualize this is to think of nuclear reactor.
Would you rather live next to a nuclear reactor which has triple safety back up generator systems to account for a power outage to stop the reactor from blowing up. Except what happens when a earthquake takes out all the back up systems like what happened with Fukushima.
Or….
Would you prefer a nuclear reactor that when the power goes off, the fuel rod drop down and the reactor stops without electricity?
This is safety by design.
Another example of safety by design is making a templating system which always escapes special characters rather than imposing cognitive load on the developer to make the right choice.