The mock that hides the bug

You mock the database. You mock the third-party API. You mock the time. Your unit tests are green, your CI is fast, your code is well-isolated. And then production breaks in a way the tests should have caught — because the mocks were lying.

When mocks lie

• The mock returns success cases that the real service doesn't actually produce.
• The mock skips edge cases (timeouts, retries, partial responses) that production hits.
• The mock's interface drifts from the real one and tests don't notice.
• The behavior under test only ever runs against the mock — never the real thing.

The honest mock

Verify against real services regularly — contract tests, integration tests, occasional end-to-end. Don't trust the mock alone for anything load-bearing. The mock is a tool for fast feedback, not a substitute for actually exercising the system.

A unit test that only ever runs against your mock is mostly testing your mock.