The public registry.

Use when git reports a detached HEAD after a checkout of a commit, tag, or remote branch, and commits made there risk being lost when you switch branches.

Use when the TypeScript compiler reports "Object is possibly undefined" or "possibly null" under strictNullChecks and the fix is proper narrowing rather than a non-null assertion.

Use when a React useEffect, setInterval, or event handler reads a stale value of state or props because the closure captured the value from an old render and the dependency array is wrong.

Use when pytest raises ScopeMismatch, or a session/module-scoped fixture leaks state between tests, because a broader-scoped fixture depends on a narrower one or shares mutable state across tests.

Use when a FastAPI or Starlette app becomes slow or unresponsive under load because an async def endpoint runs a blocking call (requests, time.sleep, a sync DB driver, heavy CPU) directly on the event loop.

Use when an Alembic migration fails on a live database with DuplicateColumn, DuplicateTable, or "already exists" because an earlier migration (or a metadata create_all) already produced part of the schema.