This will happen to you, probably in your first month. A test fails on the build server about once in ten runs. On your machine it passes every time. Someone re-runs the build, it goes green, and everyone moves on.
Do not move on. Selenium does not record video or traces for you, so your first job is to make sure a failure leaves evidence behind.
Step 1. Save a screenshot when a test fails. Add this to your teardown: