Ahhh, the mysteries of hashing. I still remember my 4 hash-failed files. I keep a hash record of everything, CRC32 for video in filenames and SHA1 for music in a FVA. I have a "main" drive and a "backup" one with the exact same files.
Not so long ago I decided to hash everything from my "backup" HDD. Out of 7500 files (~1.2 TB), 4 MKVs failed hash. The HDD seemed fine (no SMART probs or anything), and everything else in the PC was fine. I checked the files on the "main" HDD only to see it was the same 4 bad ones. No wonder, I was dumb enough when I switched my old, "main" 1.5TB with the new 2TB HDD, to copy everything from the "storage" drive and not from the disk being replaced. So both drives would have the content from a "same source". Should the backup had been corrupted and the main remained safe, the "good" files would have been saved. But in what I did if the "backup" was bad, the "main" would carry the bad files.
Lol, long story short. Whenever the files got bad, or I copied the wrong CRC32 in the files from start, I have no idea. I still have the 4 bad files that seem to be ok. I have to watch them to see how they play (I muxed them myself, and 3 of them are not available anymore to DL again).
That was my only case of bad hashes. I continued on the same system, same OS, drives, everything and never had a fail again. If I do a hash now I'll get the same results. Anyone else had bad hashes out of the blue?