you should be worried enough to keep an eye on it. Sometimes its just a single sector dies randomly, and the drive has plenty of spares to re map to (although the more that re map the slower those parts of the drive become, due to the other sectors being at the end of the drive). It could also be a sign of something wrong, and if so, you will see more bad sectors popping up and as such should RMA it if you can.
If you can back up, then do so, jsut in case.