Many SSDs ship with a small number of defective blocks from the factory (mine has 6). These are just marked as bad and replaced by some from the overprovisioned memory. As more blocks become incapable of being written to from overuse the same thing happens. Eventually you'll start losing usable capacity though. It's important to note that OCZ does not warrant the drive against normal wear, and neither do most, if not all manufacturers. This is because it is not considered a form of drive failure.