The ebuild utility contained in sys-apps/portage enables you to do fancy stuff like emerging a package applying the steps manually or fix certain things.

An example of an annoying digest verification problem:

ficus ~ # emerge -av man-pages

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild  N    ] sys-apps/man-pages-2.76  USE="-nls" 1,816 kB 

Total: 1 package (1 new), Size of downloads: 1,816 kB

Would you like to merge these packages? [Yes/No] y
>>> Verifying ebuild Manifests...

!!! Digest verification failed:
!!! /usr/portage/sys-apps/man-pages/man-pages-2.75.ebuild
!!! Reason: Filesize does not match recorded size
!!! Got: 1437
!!! Expected: 1442
ficus ~ # 

To fix this, use ebuild:

ficus ~ # ebuild /usr/portage/sys-apps/man-pages/man-pages-2.75.ebuild digest
>>> Creating Manifest for /usr/portage/sys-apps/man-pages
  digest.assumed                 3
   digest-man-pages-2.70::man-pages-2.70.tar.bz2
   digest-man-pages-2.75::man-pages-2.75.tar.bz2
   digest-man-pages-2.76::man-pages-2.76.tar.bz2
ficus ~ # 

and everything should be fine.