I think it's a question of trust and responsibility, and the client should decide where to draw those lines. Depending on the environment and context, it could be totally acceptable to trust the filesystem, but not in other cases.
Yes. It is a "storm in a glass of water", as we say. Probably the client application doesn't assume cosmic bit-flipping events for its own files, which would have equally bad consequences, either. Probably the likeliness for this happening and not being caught and corrected by the storage medium is as low as a hash or UUID collision, which everyone conveniently round down as "impossible".