From an episode of Grace & Frankie season 4, in which Frankie is erroneously registered as deceased in the Social Security database:
Grace: “You’re only legally dead.”
Frankie: “I wanna be undead!”
Grace: “That’s a whole different thing.”
Dead is an unusual root for un- to attach to — not normally a reversible process — but here F tries to use it that way, only to encounter blocking from a previously established form with its own, not entirely predictable, form.