Haskell underestimates parameter types and overestimates return types.

inference.hs

and _     False = False
and False _     = False
and _     _     = True

-- inferred type: Bool -> Bool -> Bool
-- underestimated parameter type: the function would also work called like this:
and 42 False
-- and return False; the type checker throws an error for this though.

and' _     False = 0
and' False _     = 0
and' _     _     = 1

-- inferred type: Num a => Bool -> Bool -> a
-- but no values other than 0 or 1 can be returned. The return type could theoretically be 0 | 1