The problem isn't stale data, the problem is getting data for an entirely different key. As an extreme example, if your terrible hashing algo was 'int(key) mod 10', then you have no collisions on keys "3", "45", and "99". However, if you asked for "19", you would get back the data for "99". If that data is their email address, name, address, or medical prognosis, then you just leaked PII when that data is served to a user. Worst of all, you wouldn't even know unless the customer calls up wondering why they got incorrect data.