CASE NIGHTMARE GREEN is the Laundry's codename for an "end-of-the-world" scenario in which a number of factors contribute to a higher occurrence of cross-universe activity and thaumaturgic phenomena, with potentially cataclysmic results. It is one of several scenarios collectively filed under CASE NIGHTMARE RAINBOW.
Due to the relationship between mathematical computation and the deep structure of reality, when the human population on Earth grows sufficiently large - with the result that a critical number of minds are observing reality - the local substrate of reality is weakened, allowing entities from other realities to more easily infiltrate our own, and for thaumaturgic forces to be manipulated more easily by humans. This phenomenon is amplified due to the solar system moving into a region of space that is particularly susceptible to this form of interference; this process is colloquially referred to as "the stars coming right." Additionally, recent increases in computing power have exacerbated the situation, as digital computation also contributes to the problem in much the same way that the activity of conscious minds does.