Jest tests passing individually but failing when run together

Jest tests pass when run in isolation but fail in the full suite. Tests are not properly isolated and share state.

Common causes: (1) Global state not reset — use beforeEach. (2) Module-level side effects — use jest.resetModules(). (3) Database/file state leaking — use transactions or cleanup in afterEach. (4) Mock not restored — use jest.restoreAllMocks() in afterEach. (5) Timer state — call jest.useRealTimers() in afterEach. (6) Run with --runInBand to confirm parallel issue.

Jest runs test files in parallel. Within a file, tests share module scope. Singleton modules, global state, and unreset mocks leak between tests.