One reason of
Reason: Failed on SHA256 verification
is that during creation of ebuilds some packages are fetched from external sources (for instance from adobe-flash site) and they sometimes publish new version of a package exactly under the same name.
Then next update results in a checksum error, because saved source from distfile directory is used.
To fix that you could:
rm -rf /usr/portage/www-plugins/adobe-flash/
fix it with
ebuild /usr/portage/www-plugins/adobe-flash/adobe-flash-188.8.131.521.ebuild clean prepare
Above situation should occure very rarely.