To reproduce (but not like that):
- Upload a file to use as a custom logo.
- Set ui.custom-header to that file's PHID.
- Delete the file uploaded in step 1.
- Wait about seven agonizing seconds.
- Load/refresh any Phabricator page.
- All Phabricator pages display Exception: PHID-FILE-abcd1234abcd1234abcd is not a valid file PHID.
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.