Note that no error detection code is able to detect all errors; it just lowers the probability of an error passing undetected even further. (CRCs are pretty robust in that they always detect sequences of errors with length <= N, with N depending on the particular algorithm.) With a large enough sample size, you will hit errors.

In this case it is probably memory corruption. The OS won't be able to detect such a thing unless the memory has ECC (relatively uncommon these days). It could theoretically detect it if the memory pages were checksummed and periodically verified against the checksum, but afaik no OS does so.