Web Apps took off thanks to the invariant that using them can't give you a virus. Caches can improve performance, but rely on the invariant that the cached data is the same as a freshly-computed copy. There's a long list of these in programming, and in every case, giving up the invariant means giving up the benefit.

This talk goes into practical examples of how to create and maintain invariants, using examples from the development of the Roc programming language's compiler, including how they apply to other domains like Web development and beyond.