Eventual consistency
A promise that all copies will agree if you stop changing things and wait. Until then, different readers can see different answers. It's a fair trade for speed and uptime, as long as the product was designed for it and it isn't a surprise.
An agent writes code that reads back what it just wrote, because that always works locally. Against an eventually consistent store, that code is wrong some of the time. And when two copies took different writes, something has to pick. The default is often last write wins, which means one person's change is silently thrown away.
“This store is eventually consistent, so don't read back after writing to get the new value: return it from the write. When two copies conflict, don't use last write wins for the cart. Merge the two sets of items, and add a test with two conflicting writes.”