In previous versions users were able to add Mailbox Processing Rules with invalid matches. This menu can be found by navigating to Setup > System Settings > Mailbox Processing Rules.
Whether editing an existing rule or adding a new one, at least one match option must be set to ‘Yes’. If not, validation will occur to advise the user to select at least one valid match.
Note, users can edit/access Mailbox Processing rules by navigating to Tools > Messages > Mailbox Processing Logic. The same validation applies here.