A soft error is an issue that causes a temporary condition in RAM that alters stored data in an unintended way.
Chip-level errors are rare events in which a radioactive element in the memory chip decays, releasing a proton. If the proton strikes a memory cell, it can alter the value that the cell contains. Even cosmic rays can cause this phenomenon from time to time. System-level errors may be caused by power issues, running out of specification (overclocked too far, for example) and software issues that occur when memory is written to incorrectly.
Soft errors are caused by events rather than a persistent physical condition, as is the case with hard errors. While they can be frustrating, soft errors are generally resolved by a reboot. Sometimes, however, these issues recur because of a persistent software issue or a problematic setting. Without careful testing, a soft error caused by persistent issues can be misinterpreted as a hard error.
Attempts to write to protected memory space reserved by another program or the operating system always create errors and are a typical cause of a blue screen of death (BSOD).