I upgraded PostgreSQL from 7.4.x to 8.0.x, using "emerge postgresql". When I returned to it, I found postfix was refusing incoming connections. So I tried "postfix stop" at the commandline, whereupon it complained of a missing libpq.so.3. "ldd /usr/sbin/postfix" confirmed "libpq.so.3 => not found". Re-emerging postfix (exactly the same version I already had) fixes it. But that effectlively leaves anyone affected without a mailserver until the problem is detected ("where's my mail?"), diagnosed, and fixed! Reproducible: Always Steps to Reproduce: 1. 2. 3.
That's why we have revdep-rebuild.