An agent-memory bug can look nothing like one. It looks like confidence: the agent remembers an old address, keeps a preference the user changed months ago, or retrieves a fact that should have expired weeks back. Nothing crashes. The answer is just wrong - and the reason is buried in a store that kept growing because every write looked harmless at the time.
This article is about that layer: what goes wrong after agent memory starts working. Five failure modes show up often enough to be worth checking any store against, so each one below carries the question to put to yours. They're not equally bad - three of them are correctness problems, the fourth is a security one - and I'll be honest about which parts a memory layer can actually fix versus merely make visible.
One bias up front, so you can discount for it: Engrava (the library I work on) has no LLM in its write path. That's relevant because when nothing generative runs on write, what the store does is decided by code you can read - so I can be specific about where it breaks, and, where it can, design against that in the data model rather than hoping a bigger model smooths it over. Where it can't, I'll say so.
1. Hallucinated metadata
