Sorry, actually I did all that a week ago (but was too lazy for feedback).
I do find a few 412/416 errors in the logs (I made a number of backups of the
whole folder to check things out) so if just one error can corrupt dozens of
unrelated files then I can't rule that possibility out. 

Well note that my argument still holds in principle ;)

Since I already wrote a script to check for unloadable images I'll launch the
whole thing again when 45-3 will have made its way in Debian/testing.