To reproduce (but not like //that//):
1. Upload a file to use as a custom logo.
2. Set `ui.custom-header` to that file's PHID.
3. Delete the file uploaded in step 1.
4. Wait about seven agonizing seconds.
5. Load/refresh any Phabricator page.
6. All Phabricator pages display **Exception: PHID-FILE-abcd1234abcd1234abcd is not a valid file PHID**. {F623084}
While I agree that invalid PHIDs are a crime, I think that revoking all access all Phabricator pages is probably a cruel and unusual punishment. One should probably be able to, for example, go back and correct the mistakes they have made.