The following program is accepted by ghc even though it is clearly wrong. The type family T is closed so T Bool cannot ever be reduced, so it must be a type error.

{-# LANGUAGE TypeFamilies #-}
module Err62 where

type family T a where
    T Int = ()

x :: T Bool
x = undefined

f :: T Bool -> Bool
f _ = True

y :: Bool
y = f x