Amazon's shopping cart was built on a strange rule: a write must never fail.
No single leader. Any replica accepts your "Add to Cart." If the right server is unreachable, a neighbour takes it and passes it on later.
The price: two servers can hold two versions of your cart. Amazon's merge rule was simply — keep everything.
Which means a deleted item can quietly come back.
That's not a bug they missed. It's the receipt.
Because one failure loses a click. The other loses the customer.