An invisible rain
Particles of enormous energy constantly fall on Earth from space. They were discovered by the Austrian physicist Victor Hess during balloon flights in 1912. In the atmosphere they break up into showers of further particles, and some of these pass through buildings and computers too.
When a single bit changes
A bit in memory is a tiny electric charge. If a particle flies through a chip, it can change it, turning a 0 into a 1. In 2003, in the Belgian town of Schaerbeek, an electronic election count credited one candidate with 4,096 extra votes, exactly matching a single flipped bit (2¹²). The error was caught because she had more votes than were possible.
Mario and aeroplanes
In 2013 players of Super Mario 64 noticed that Mario suddenly moved higher up during a speedrun. The most likely explanation is precisely a single bit flip. The higher up you go, the more radiation there is, which is why aircraft, and especially satellites and space probes, have to account for it.
How to defend against it
Servers, supercomputers and quality NAS units use ECC memory. For every block of data it stores extra check bits, correcting one error itself and detecting two. In an ordinary computer without ECC, such an error mostly goes unnoticed altogether; other times it causes a rare program crash or a corrupted file.