"test error" or "misprediction" aren't very applicable terms for RL, though. is it a "test error" if the LLM suggests a bad chess move? is it a "misprediction" if the LLM can't find a mating sequence? are those "failures to forecast out-of-sample inputs," or only if the LLM suggests an illegal chess move or call an API that doesn't exist?

I say that a model "hallucinates" only if it specifically presents false information as fact - e.g. that Bolognium is element 115, or that chocolate milk comes from brown cows. giving irrelevant or undesired answers isn't hallucination or confabulation. but I don't think your terms are precise enough for that nuance.