This is something very important that we usually don’t realize. There is always this checklist or requirement specification document that we are so busy in verifying that we often don’t find out about unwanted/unknown behaviors. If you are fortunate, you might stumble upon it while testing. Otherwise, it is brought to your attention when something breaks or behaves unusually when the user is using your app or software.
I always like to drift away from the (features/requirements) checklist while testing. Testers have this sense where they can smell the areas where there might be something “unwanted” or “unknown” happening. It is always good to explore those areas before (or during) verifying that checklist. Your superiors/clients will thank you for it.