If you install an older version of a wired-in package (e.g. template-haskell) and then try to use it, GHC will use the newer one as the "wired-in" package rather than the older one.

This happened when trying to build aeson-0.4.0.0 with ghc 7.4: aeson requires template-haskell-2.6.0.0, but GHC ships with 2.7.0.0. Cabal installed template-haskell-2.6.0.0, but while building aeson against it we get:

Data/Aeson/TH.hs:181:1:
    Bad interface file: /Users/tibbe/.cabal/lib/template-haskell-2.6.0.0/ghc-7.4.0.20111213/Language/Haskell/TH.hi
        Something is amiss; requested module  template-haskell-2.6.0.0:Language.Haskell.TH differs from name found in the interface file template-haskell:Language.Haskell.TH

the bug is that findWiredInPackage has picked template-haskell-2.7.0.0 to be the wired-in package:

wired-in package template-haskell mapped to template-haskell-2.7.0.0-inplace

whereas we wanted to use template-haskell-2.6.0.0.

Not immediately obvious what the fix should be, so I'm making a ticket for this. The workaround is to update the dependency in aeson to allow template-haskell-2.7.0.0.