The lost-update bug that quietly corrupts homegrown wallet balances — and the five disciplines we used across PayWithToken to make money movement correct under concurrency. There is a bug that lives in a large share of the world's homegrown wallet systems. It doesn't throw an error. It doesn't show up in tests. It surfaces months later as a balance that is quietly, inexplicably wrong — and in a payments system, a wrong balance is either a customer who has lost money or a company that has give...