Defensive design is the practice of planning for contingencies in the design stage of a project or undertaking. Essentially, it is the practice of anticipating all possible ways that an end-user could misuse a device, and designing the device so as to make such misuse impossible, or to minimize the negative consequences. For example, if it is important that a plug is inserted into a socket in a particular orientation, the socket and plug should be designed so that it is physically impossible to insert the plug incorrectly. Power sockets are often keyed in such a manner, to prevent the transposition of live and neutral. They are also recessed in the wall in a way that makes it impossible to touch connectors once they become live.
Defensive design in software engineering is called defensive programming. Murphy's law is a well-known statement of the need for defensive design, and also of its ultimate limitations.
Implementation decisions and software design approaches can make software safer and catch user errors.Code that implements this is termed a sanity check.
Many electrical connectors apply this principle by being asymmetric.Alternatively, USB-C plugs are mechanically but not electrically symmetric, but achieve an illusion of symmetry resulting from how devices respond to the cable, and hence can be plugged in either of two ways. Software makes the plugs and cables behave as though they are symmetric.