ECC errors should be rare. I would expect less than one a week. If they always occurs on the same RAM module, that module is defective. If memory modules are not defective, the only cause of these errors would be radiation, such as cosmic rays. A cosmic ray passing through the memory chip can generate enough charged particles to change one bit in the memory. The ECC (Error Correcting Circuitry) can correct a single bit in a word by using extra bits that were written along with the data when that word was written. If more than one bit is wrong, it can't be corrected, but will be detected, and OSX will kill the Application, or restart OSX. If you are getting lots of RAM errors, you have bad RAM, or you are keeping your used nuclear reactor fuel rods too close to the computer.
