Bruce Schneier on worst case thinking
Worst-case thinking means generally bad decision making for several reasons. First, it’s only half of the cost-benefit equation. Every decision has costs and benefits, risks and rewards. By speculating about what can possibly go wrong, and then acting as if that is likely to happen, worst-case thinking focuses only on the extreme but improbable risks and does a poor job at assessing outcomes.
Bruce covers the idea in depth, but for me the problem with worst case thinking is that is can be used to justify doing nothing. By focusing purely on the extreme possible downside it forgets the value of the benefits, AND forgets that doing nothing also has a cost.