Re: MonadFail proposal (MFP): Moving fail out of Monad

Thanks for putting this together. The proposal says: "As a consequence, in current Haskell, you can not use Monad-polymorphic code safely, because although it claims to work for all Monads, it might just crash on you. This kind of implicit non-totality baked into the class is terrible." Is this actually a problem in practice? Is there any code we can point to that suffers because of the current state of affairs? Could it be included in the proposal?