I wouldn't worry to much about it to be honest. If you are not getting Blue Screens or system failure it really probably wouldn't affecting you too much. Worst case scenario, a memory module were to fail, they are easily replaced and with the cost of them so low right now, they are easily expendable.
Most likely you have a bad section of RAM that throws an error on your testing software. The reason for it not effecting your system to much is because it is rarely/if ever used (probably located on the higher end of your 12GB of memory)