Re: Strange (cosmic ray?) corruption.

["Gareth Evans" <donotspam@fastmail.fm>]

Have you over/under-clocked or otherwise adjusted your CPU settings?

This "solved" issue which seems to be similar is put down to processor core instability under certain conditions:

https://bbs.archlinux.org/viewtopic.php?id=290093

As the last comment mentions, I was also wondering about the possibility of thermal issues.  

Might a smartmon or memtest test be worthwhile?

You don't seem to be dumping from read-only snapshots afaics, but that these particular files might be changing between checksum and compression (or whichever comes first) seems an unlikely spanner in the works.

HTH
Gareth