The following code causes the following panic:

{-# LANGUAGE TypeFamilies, GADTs #-}

type family F a

data G1 a where
  G1C :: F Int

data G2 a where
  G2C :: F Int

ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.0.20120810 for x86_64-apple-darwin):
	compiler/typecheck/TcTyClsDecls.lhs:1081:5-62: Irrefutable pattern failed for pattern Data.Maybe.Just subst

Admittedly, the code causing the panic is silly, but it happened to me as I was refactoring. Having two separate GADTs with the wrong return types seems necessary to cause the panic.