Hydration mismatch

The client render producing different output from the server render, usually because the component read something that only exists in one of the two places: the current time, a random value, `window`, or a user's locale. The framework has to discard its assumptions and re-render, which throws away the benefit of server rendering for that subtree. Treat every mismatch warning as a real bug, because the ones that do not warn are the ones that show the wrong content.