If you know something to be dangerous, then you can prepare for it. You can take steps to account for the danger. You can be ready to take it on or brace for it. In a way, the danger is no longer so dangerous.
But if the danger is neither seen nor expected, it can blindside you. Even something minor, if not accounted for can trip you up or even set off a chain reaction. We should take care to be as aware of all dangers, however minor, and intelligently prepare.