Filing this bug as requested in bug 149062 comment #77. The make.conf man page should mention, under the PORTAGE_TMPDIR topic, that pointing PORTAGE_TMPDIR to a directory under PORTDIR is not a good idea / not supported / advised against / however you want to word it.
Created attachment 321540 [details, diff] patch for make.conf man page This documentation bug still exists in portage-2.1.11.9. Here's one possible patch to fix it.
Thanks, this is in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=fab71caabba4e57ffcbed0651abcee014d1c960f
This is fixed in 2.1.11.11 and 2.2.0_alpha122.