Proxy-maint just hit an ebuild where proxied maintainer used 'doins' to install files recursively into /. Aside the fact that this caused wrong permissions, I think that PMS doesn't specify any default path for doins. Please make Portage error out if 'doins' is called with no path set by earlier 'insinto' call.
Here's a list of current offenders (probably not comprehensive): dev-util/intel-ocl-sdk-2.0.31360 media-gfx/evoluspencil-2.0.5 sci-physics/geant-data-4.10.00 sci-physics/geant-data-4.10.01 sci-physics/geant-data-4.9.6_p02 sci-electronics/systemc-2.2.0-r2 sci-libs/spqr-1.2.3-r1 sci-visualization/xyscan-3.3.1 # Relies on the doins() call in linux-mod_src_install(). sys-fs/aufs4-0_pre20160627 sys-fs/aufs4-0_pre20160219 sys-fs/aufs4-0_pre20160223 sys-fs/aufs3-3_p20160627 sys-fs/aufs3-3_p20160223 sys-fs/aufs3-3_p20160219 # Bug, should be using dodoc() instead of doins(). dev-libs/gecode-3.7.3
This looks like a good candidate for an eqawarn QA Notice.