A parity error is registered when an accidental change in the value of the data saved on the memory is observed. Parity errors can be of 2 types- the soft parity errors and the hard parity errors.
Soft parity error can happen because of the following reasons:
a. An Electro-Magnetic Interference from some device sitting very close to this device.
b. An Electro-Static Discharge due to improper handling of the device.
c. External cosmic radiation from space, such as solar flares.
d. Due to radiation caused by trace radioactive material present in any substance that happen to come near the device.
Soft parity errors are rare, transient errors that can occur on any device which has been working perfectly fine.
Hard parity errors occur when there is a hardware failure on a chip or board. They are generally observed repeatedly in a particular region of the memory.
Soft parity errors are more far more frequent than hard parity errors. Therefore, you should wait for a second parity error before considering any hardware replacement. If the first crash was caused by a failing hardware, then the crash would happen again. Otherwise, it must have been a transient soft parity error and may be ignored.
Refer http://www.cisco.com/en/US/products/hw/routers/ps341/products_tech_note09186a0080094793.shtml for more details.