The following doesn't compile with GHC 7.8.3, but works with GHC HEAD. I couldn't find a matching ticket, so I don't know if this was fixed knowingly or not...

{-# LANGUAGE AutoDeriveTypeable #-}

import Data.Typeable (Typeable)

data T1 = C1 Int
     deriving (Eq,Ord)

tvoid :: Typeable a => a -> IO ()
tvoid _ = return ()

main :: IO ()
main = tvoid (C1 0)

1. ..fails for GHC 7.8.3 with

    No instance for (Typeable T1) arising from a use of ‘tvoid’
    In the expression: tvoid (C1 0)
    In an equation for ‘main’: main = tvoid (C1 0)

I'm marking this with high priority, as it makes -XAutoDeriveTypeable unusable on GHC 7.8.3 as it stands.