Cross-session / cross-tenant memory bleed: shared memory / cache without per-user isolation, and I hand one person's data to another
In one sentence in March 2023 a redis-py concurrency race in ChatGPT let some users see others' chat titles and the first message of a new conversation, and about 1.2% of active Plus users had their name / email / billing address / last-four card digits exposed in a roughly 9-hour window (OpenAI's official postmortem). Conclusion first: cross-session isolation is the system's responsibility, not "I'll be careful not to mix things up" — scope every cache / memory / session by user, with concurrency safety + ownership checks + auditing.
Multi-tenant RAG retrieval leakage: vectors aren't anonymous, and filtering by user isn't always isolation
In one sentence: in multi-tenant RAG, two intuitions both fail — "vectorized means anonymized" and "filtered by user means isolated." In some studied embedding settings a vector can be inverted back to approximate original text (including PII), so don't treat it as anonymization; retrieval ranks by similarity, not permission, so if the ACL runs after retrieval instead of before it, the most relevant private chunk may come from another tenant; and a single boundary bug in a shared cache / long-term memory can carry one user's private context into another's session.