A passing test suite can be 100% green while production silently ships the wrong total on every checkout. This is why unit tests lie — and what senior engineers test instead: real integration, property-based testing, observability, and contracts.

247 tests passing in 800 milliseconds tells you almost nothing about whether the system actually works. A green unit test only confirms that one bolt, in isolation, is fine — while the engine is on fire. The moment you mock the database and stub the API, you stop testing your application and start testing your imagination of how those things behave. This video breaks down the three things seniors trust instead of the green bar, with real Python and TypeScript examples