One of the most classical and verified principles of physics is that entropy never decreases. E.g. gas occupying half of a 1L container will quickly disperse quite-evenly throughout the container. It seems however, that QM easily allows momentary violations of this principle. For instance (the classical QM-for-the-laymen idea) is that such a collection of particles (gas in a 1L container) could spontaneously arrange itself into half of the container, with some small probability. If that happened, wouldn't it be a drastic decrease in entropy?