When I worked on S3, I was briefly responsible for reporting waste in the system. The basic equation was [Total Capacity of Hard Drives] - [# of bytes customers are paying for] * [Replication factor] = Waste

One week as I was preparing the report, it was clear something had gone haywire. Waste was roughly equal to total capacity. So either we'd lost all of our customers overnight, or there was a bug. Turns out the legacy billing system was using a long to count # of paid bytes and this had overflowed. So it does happen.