GHC accepts this garbage:

type family F2 :: forall k. k -> Type
data SBool :: Bool -> Type
data Nat
data SNat :: Nat -> Type
type instance F2 = SBool
type instance F2 = SNat

The family F2 should have an arity of 0, meaning that only one instance is possible -- and the RHS of that instance must have kind forall k. k -> Type. In other words, even accepting only one of the instances above is hogwash.

This is from ticket:11719#comment:161416, but you don't have to read that to understand this.