For example, see bug 558856 comment 0:
> vdb entries did not get adjusted for packages using subslot deps. For
> 0=[abi_x86_64(-)] sys-process/procps:0/0= dev-libs/libaio:0/0=
> >=sys-apps/sed-4 >=sys-apps/texinfo-4.7-r1
> >=sys-libs/readline-4.1:0/0=[abi_x86_64(-)] !dev-db/mytop
> >=dev-libs/libpcre-8.35:3/3= !dev-db/mysql !dev-db/mariadb-galera
> !dev-db/percona-server !dev-db/mysql-cluster dev-db/mysql-init-scripts
> virtual/perl-Getopt-Long dev-perl/TermReadKey virtual/perl-Term-ANSIColor
> This is wreaking havoc on users' systems when they go to update. Autounmask
> suggests allowing unstable on ncurses-5.9-r101, but not on 6.0, the dep tree
> explodes. I'm sure I and others will have more details to add but I wanted
> to get this bug open finally. Been seeing problems in #gentoo all day.
> Right now were are suggesting users force-rebuild all packages with a dep
> like sys-devel/ncurses:5 recorded in /var/db/pkg/*/*/RDEPEND, other options
> like a forced sed on /var/db/pkg seem a bit scary.