Validity failures: wrong or outdated facts

The SSGM paper separates two validity problems that are easy to confuse:

• Memory hallucination: the agent stores a fabricated fact as if it were true. Because it is now in memory, it gets retrieved and acted on later.
• Temporal obsolescence (staleness): the stored fact was correct once but the world has changed. This is not a hallucination; it is just old.

The "Practical Guide" calls staleness probably the most common failure. The outside world changes (addresses, device states, user preferences) but the stored memory does not. Long-lived agents will happily act on data from 2024 in 2026. The classic symptom is an agent insisting on an outdated fact about the current date or who holds an office.