Today, we encountered a very critical bug that would be missed with unit testing and integration testing, which changed our whole approach towards how we view testing. It was a simple scrollbar not being able to scroll 90% times only worked 10% if a user did very specific thing. Overall chance of getting noticed in beta test would have been 2% but as a startup this could have been our end, because unless user added more than 5 files one could not test the scenario, which is the avg. for any trial user. But a real user would have noticed this and would have left for another company. When we started we hadn't hoped this would be an issue but here we are.