Believing the worst

Shamelessly stolen from Overcoming Bias:

In 1983, NASA was planning to bring back Martian soil samples to Earth. Contaminating the Earth with alien organisms was an issue, but engineers at Jet Propulsion Laboratories had devised a "safe" capsule re-entry system to avoid that risk. However, Carl Sagan was opposed to the idea and explained to JPL engineers that if they were so certain [...] then why not put living Anthrax germs inside it, launch it into space, then [crash the capsule back to earth] exactly like the Mars Sample Return capsule would.

The engineers helpfully responded by labeling Sagan an alarmist and extremist. But why were they so unwilling to do the test, if they were so sure of their system? The answer is probably they feared that if the test failed, their careers would be over and they would have caused a catastrophe. But an out of control Martian virus, no matter how unlikely, would have been equally a catastrophe. However, that vague threat didn't concentrate their minds like the specific example of anthrax.

Imagine for a moment that those engineers had been forced to do Sagan’s test. Fear of specific disaster would have erased their overconfidence, and they would have moved from 'being sure that things will go right' to 'imagining all the ways things could go wrong' – and preventing them. The more dangerous the test, the more the engineers would have worked to overcome every contingency.