CAP theorem
When a network splits your copies of the data apart, you have to choose. Either refuse some requests, so nobody gets a wrong answer, or answer them all, and accept that some answers are out of date. You can't have both while the split lasts.
It's a staple of interviews, and an agent can recite it confidently, including the slogan that's wrong. What matters is the question it forces on you: for this one piece of data, during an outage, would you rather show an error or show something stale? A bank balance and a like count have different answers.
“For each kind of data here, tell me what the user sees during a network partition: an error, or possibly stale data. Balances must refuse. The product catalogue may be stale. Don't just call the database CP or AP. Say what happens to a read and a write.”