Yes, that can easily happen. I have most often seen it when a package
is split. For example:
T1: You install usefulpkg-1.0, which depends on sometool-1.0
T2: sometool version 2 is released, and pkgsrc decides to
split it into sometool-base-2.0 and sometool-2.0,
with sometool-2.0 depending on sometool-base-2.0.
sometool-base-2.0 is marked as conflicting with sometool-1.0.
pkgsrc changes usefulpkg-1.0nb1 to depend on sometool-base-2.0,
where usefulpkg-1.0 used to depend on sometool-1.0.
T3: You update your pkgsrc tree, and run pkg_rr.
pkg_rr wants to upgrade usefulpkg from version 1.0 to 1.0nb1.
pkg_rr needs to build sometool-base-2.0 as a dependency for
usefulpkg-1.0nb1
sometool-base-2.0 can't be installed, because it conflicts with
sometool-1.0.
T4: You figure out what's wrong, manually delete sometool-1.0,
run pkg_rr again, and now it works.
--apb (Alan Barrett)