[Haskell-cafe] A question about "monad laws"

Tutorials about monad mention the "monad axioms" or "monad laws". The tutorial "All About Monads" says that "It is up to the programmer to ensure that any Monad instance he creates satisfies the monad laws". The following is one of the laws. (x >>= f) >>= g == x >>= (\v -> f v >>= g) However, this seems to me a kind of mathematical identity. If it is mathematical identity, a programmer need not care about this law to implement a monad. Can anyone give me an example implementation of monad that violate this law ?