We have an errata_systems_list that represents a saved set of selected systems. Right now someone can select systems for one errata, update their list, go to another errata that is not applicable for the systems they have selected, and they will still technically be selected. This allows someone to schedule irrelevant errata for systems, even accidentally.
Users can no longer schedule errata for unaffected systems and unaffected systems do not appear on the confirm page. However, there still can be undisplayed-but-selected systems on the SystemsAffected page.
Making new bug for sets. Users can no longer schedule errata for unaffected systems, so I consider this bug done.