The Following code

f :: (Show a, Eq a) => a -> String
f x = show x `const` g x

g :: (Eq a,Show a) => a -> Bool
g x = x==x `const` f x

gives this error:

    Couldn't match expected type `Eq a' against inferred type `Show a1'
    When matching the contexts of the signatures for
      f :: forall a. (Show a, Eq a) => a -> String
      g :: forall a. (Eq a, Show a) => a -> Bool
    The signature contexts in a mutually recursive group should all be identical
    When generalising the type(s) for f, g
Failed, modules loaded: none.

but it compiles fine if i swap Show and Eq in the context of either f or g. The order of typeclass constraints usually doesn't matter so it's very surprising that it does here.