The most dangerous idea I have come across recently is the idea that we understand plutonium. Plutonium is the most complex element in the periodic table. It has six different crystal phases between room temperature and its melting point. It can catch fire spontaneously in the presence of water vapor and if you inhale minuscule amounts you will die of lung cancer. It is the principle element in the "pits" that are the explosive cores of nuclear weapons. In these pits it is alloyed with gallium. No one knows why this works and no one can be sure how stable this alloy is. These pits, in the thousands, are now decades old. What is dangerous is the idea that they have retained their integrity and can be safely stored into the indefinite future.
The idea that we understand plutonium