Any number of conditions could make it fail, but most especially GC.
It is almost always better to explicitly model a property when you want it. Uniqueness based on construction, for example, can be modeled with a monad and protected by an ADT.
> module Unique ( HasUnique(..), Unique, newUnique, openUnique) where
> class HasUnique m uid where
> newUniqueID :: m uid
>
> data Unique uid a = U !uid a
>
> newUnique :: (HasUnique m uid, Monad m) => a -> m (Unique uid a)
> newUnique a = newUniqueID >>= \ uid -> return (U uid a)
>
> openUnique :: (Unique m a) -> a
> openUnique (U _ x) = x
>
> instance (Eq uid) => Eq (Unique uid a)
> instance (Ord uid) => Ord (Unique uid a)