In the case of URLs, you can have /questions/:id/:slug, and then load the question by :id, but if :slug doesn't match the slug you've got in the database, then return 404.
Sure, but that means that you have a unique key combination of id + slug for all intents and purposes. The argument for having the ID is that the slug can change and you can redirect. If you don't redirect, that argument falls flat, and if you redirect you're easier to enumerate.