It comes up durring hiring processes. Pretty common to have someone ask, "Have you entered into any legal agreement that would prohibit you from working with us or have any conflict of interest? If so please explain."
For anyone who hasn't signed an employment contract in California, it'll have the boilerplate noncompete they use everywhere but it will also have an addendum page that says basically "the noncompete back on page x doesn't apply to you so ignore it".