When does a difficulty become UNDESIRABLE?

When the learner cannot overcome it — material far above your level, or that produces failure with no learning, is an undesirable difficulty. Harder helps only up to the point where you can still succeed with effort.

The word "desirable" is load-bearing and easy to forget. The benefit comes from successful effortful retrieval, not from struggle for its own sake. If a task is so hard you simply fail — no correct answer generated, no productive engagement — you get the frustration without the durability payoff, and you may even reinforce errors. The design goal is a level of challenge you can meet with effort: hard enough that success is not automatic, easy enough that success is reachable. This is why beginners should build basic competence before piling on difficulty.