Hard drives store things with varying forms of ECC. Each sector has an ECC field, allowing it to detect many errors and automatically correct some.

This isn't a replacement for something better, it just allows simple bit errors to be corrected automatically by the drive. The problem is that it's not really obvious when it's happening, and you only notice when it can't fix something. Drives eventually throw a SMART error when it's had to do too many corrections though.