Cosmic rays can cause bit-flips (as sibling commenter mentioned). But there's also other interesting ways that bits can flip in memory. For example, Sun had an

Cosmic rays can cause bit-flips (as sibling commenter mentioned). But there's also other interesting ways that bits can flip in memory. For example, Sun had an interesting issue with their SRAM where their manufacturer had accidentally doped the SRAM with radioactive boron. This resulted in alpha particles being emitted that caused corruption in the SRAM (which then caused Solaris to crash because they had parity bit checking).