cosmic ray

Electrical or magnetic interference inside a computer system can cause a single bit of DRAM to spontaneously flip to the opposite state. It was initially thought that this was mainly due to alpha particles emitted by contaminants in chip packaging material, but research ^[5] has shown that the majority of one-off ("soft") errors in DRAM chips occur as a result of background radiation, chiefly neutrons from cosmic ray secondaries which may change the contents of one or more memory cells, or interfere with the circuitry used to read/write them.