What are the most common Jest mistakes?

Mistakes worth checking first

• Forgetting to await an async assertion, so the test finishes early
• Mocking so much that the test only verifies wiring
• Sharing a mutable object across tests without rebuilding it
• Using a fixed sleep instead of waiting for a condition
• Running jest -u without reading the snapshot diff
• Spying on a method and never restoring it
• Asserting on implementation details such as hook call counts

test("loads the profile", async () => {
  await expect(loadProfile("u1")).resolves.toMatchObject({ id: "u1" });
});

test("shows a loading state first", () => {
  render(<Profile id="u1" />);
  expect(screen.getByRole("status")).toBeInTheDocument();
});

Most of these mistakes share a root cause: the test is observing the wrong layer. If a test reaches into module state, internal hooks or exact call order, it is coupled to the implementation. If it observes user-visible behaviour at a stable boundary, many of the pitfalls disappear.