As prerequisity, install any package which has as runtime dependecy something like "<net-www/apache-2.1.0" along with, let's say, "net-www/apache-2.0.55-r2". Now comes the point: If you unmasked "net-www/apache-2.2.0" and ran "emerge world", apache will be upgraded to 2.2.0. Portage does not respect that an entry in the world-file, the one with "<net-www/apache.2.1.0", does not work with the newer version! The expected behaviour is, that portage either warns (!) "XY is blocking apache-2.2.0!" or that simply apache is left untouched/not upgraded.
Known(In reply to comment #0) > The expected behaviour is, that portage either warns (!) "XY is blocking > apache-2.2.0!" or that simply apache is left untouched/not upgraded. You'll get a downgrade next time you run emerge -uD world, and upgrade next time... and so on. Known issue for a long time, deps like <net-www/apache-2.1.0 don't work. You need to use package.mask or !>net-www/apache-2.1* in the ebuild. Cannot find the duplicate right now.
*** This bug has been marked as a duplicate of 48195 ***