A flaky test passes and fails on the same code. Nothing changed between the two runs except timing, ordering, or something outside the test that nobody was thinking about. It is worse than a test that always fails, and that is not a figure of speech.
Source: [Dev.to](https://dev.to/session_replay/what-is-a-flaky-test-why-it-costs-more-than-the-bug-it-was-meant-to-catch-583a)