If a bit flips after checksumming, whether in the data or checksum, the file can either be fixed automatically or marked corrupted for manual restoration. But

If a bit flips after checksumming, whether in the data or checksum, the file can either be fixed automatically or marked corrupted for manual restoration. But if the problem is the RAM or CPU, the bit could also flip before checksumming. Then the incorrectly written data will have a matching checksum and the corruption can't be detected. That's why ECC is important.