I had fun once diagnosing a bad bit in a drive's cache memory. Writes would go out, and sometimes come back with a bit set. All the disk-resident CRCs in the world won't help if your data is mangled before it makes it to the media.

File systems with end-to-end checking are good. (These can turn into accidental memory tests for your host, too, and with a large enough population you'll see interesting failures).