The difficulty in explaining observations of Saturn’s motion in the context of the geocentric model was a crucial stepping-stone that eventually led Copernicus to propose the heliocentric model. This paper analyses approaches for building safer control systems related to this analogy: detecting mismatches between a controller’s view of the environment and its actual state can greatly prevent accidents. We survey existing work on the topic and look for ways of systematically comparing, assessing, and devising effective monitoring solutions, from the detection of a simple component failure to the circumvention of flaws in the system requirements specification.