[booi]

ZFS and other checksumming file systems can detect bit rot in data at rest. When data is read back, that sector is checksummed and compared against when it was written before returning the request.

You can periodically scrub the entire pool to find and even fix these issues (in a pool with redundancy)

[legulere]

The bit rot could have still happened during writing though, if the bitrot is found in the first scan.

[huhtenberg]

Sure. However the main purpose of scrubbing is to flush out deteriorating media and to prompt the drive to relocate salvageable sectors and to report completely dead ones.