Your electronic design is haunted by intergalactic gremlins. The Earth is constantly bombarded by high-energy particles from stars all throughout the galaxy. These charged particles consist of subatomic particles, such as neutrons, electrons and muons, and fully-ionized atomic nuclei traveling at relativistic speeds. When these cosmic rays strike the oxygen and nitrogen molecules in our atmosphere, they leave a shower of high-energy photons and elementary particles in their wake.
A portion of those high energy particles can make it all the way through the atmosphere to the surface of the Earth. If a cosmic ray, or one of its byproducts, passes through your printed circuit board, it can either damage PN-junctions, temporarily change the logic-level thresholds of digital inputs and digital outputs, and temporarily or permanently flip bits and entire words of memory in your microprocessor or microcontroller.
While the events are rare on the surface of the Earth, the consequences of a Cosmic Ray strike can be deadly. Cosmic Rays are thought to be responsible for lightning strikes, airplane accidents, runaway automobiles, and perhaps even erroneous election results.
So, just how susceptible is your design to a cosmic-ray event? Well, absent two-meters of leaded-concrete shielding, a high-energy cosmic ray that is headed for your design will hit it. But not all cosmic rays are that energetic — so luckily, there are some design elements that can attenuate the frequency of impacts. But cosmic rays cannot be completely stopped by any sort of commercially available enclosure or shielding.
Unfortunately, statistics and luck determine whether or not your design will be struck by a primary or secondary cosmic ray, and your component choice and circuit design skills will determine whether or not your device will malfunction or survive the event.