That’s a great challenge–I’ve been thinking a lot about this since OP–and I can think of articles or chapters that are hard to find.
But the only reference I’ve ever searched for and couldn’t find (as opposed to finding it behind a paywall or just hard to get) was an unpublished paper–so that doesn’t really count. I can’t think of any reference with a DOI that haven’t been able to track down (one had an easy-to-fix typo).
Field: Psychology