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.