Is it possible that just one bit switches so my file shows me a letter "Q" instead of a "S"

Solution 1:

It's so rare we see a genuinely interesting question on this site, so thank you first of all.

I think what you're seeing there is indeed a single-bit error, amazing you could spot it to be honest but you're correct in assuming that the second-least-significant-bit has been switched (assuming you're using ASCII anyway).

As for checksums etc. when it was written to the disk it's likely it will have been verified as fine - I'm pretty sure this problem has developed afterwards via a simple magnetic-leakage error. But you're right, there are encoding checks done, it varies from manufacturer but there's probably a error somewhere saying 'this looks a bit odd' - but what option does your IO chain have available? deny you the whole block? I'm going to assume this is a single non-RAIDed disk as they RAIDed disks tend to have more options available to them when they detect errors.

It's an odd one, though this kind of thing probably happened multiple times a second across the world.