How about...

You are attempting to load a Saga by a property that is not marked with the [Unique] attribute. Sagas depend on consistency and without this attribute, RavenDB cannot guarantee that the correct saga data will be loaded. It is recommended that you determine which Saga data property uniquely identifies the Saga (OrderId, CustomerId, etc.) and mark that property with the [Unique] attribute. However, it is important to note that for Saga data that is already "in-flight", adding the [Unique] attribute will cause those Sagas to not be found. See {shortlink to documentation that paraphrases @thecolour 's SO question}