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I actually think the Known Knowns could be the most hazardous category.
The other sections allow us to acknowledge the fact that we don’t know everything, and encourages us to intentionally seek out that information.
However, icebergs are constantly changing. They melt, they change shape, and sometimes they even flip over. It is common for documentation not to be kept up to date and requirements to change. This can lead to confusion when developing and changing the application, and severely impact our ability to continue our testing.
We need to keep on track with the Known Knowns, check they are still relevant and correct. People often talk about the limitations of Automated Tests, how they only check the same thing over and over again. That isn’t a bad thing. It gives us that early warning system when something unexpected has changed.