A test can pass and still be a problem. Let me show you the two ways that happens, because you will meet both.
Problem 1: the test races the app.
This is the fixed pause from the waiting lecture. A test with waitForTimeout(400) passes on your fast laptop and fails on the slower build server, maybe once in ten runs.
What makes this dangerous is not the failure. It is what the team learns. They see red, they re-run the build, it goes green. After a month, everyone re-runs red builds without looking at them.