Fabrication of modern integrated curcuits involves building up multiple, overlapping layers of semiconductor (various types) or metal through repeated etching, plating, and doping. The proximity of these many layers in the right 3-D configuration make components, such as transistors, connected up to make a specific circuit. If a cosmic ray goes zapping through such an integrated circuit, it can knock off electrons and charge up components in some places and, for example, turn a 1 into a 0, or turn on a transistor that doesn't exist on any circuit diagram, but which exists in the 3-D pattern of layers if the wrong spot is zapped. The latter are called phantom transistors, and they can be hard to turn off again. Sometimes they will discharge on their own. Sometimes you have to remove power to the circuit before the phantom transistor will go away. And in the worst cases, the IC would require re-annealing to get it back to it's design state.
