Suppose you protect a piece of data with a checksum. The data gets corrupted and the checksum no longer matches. That's a detectable error. But if you're unluck

Suppose you protect a piece of data with a checksum. The data gets corrupted and the checksum no longer matches. That's a detectable error. But if you're unlucky, the checksum of the corrupted data matches the original checksum. That's an undetectable error.