The following code causes a panic:

{-# LANGUAGE FunctionalDependencies, PolyKinds #-}

class Baz a b | a -> b where
  bar :: a -> ()

instance Baz Bool Bool where
  bar _ = ()

foo = bar False

The panic reads:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.7.20121031 for x86_64-apple-darwin):
	ctEvTerm: derived constraint cannot have id
<<details unavailable>>

The panic does not happen without -XPolyKinds.