It seems that some ebuilds are attempting globbing in DISTDIR and similar ops. While this partially works in Portage because of shadow dir use, this is unreliable and a very bad idea. Common mistakes: 1. cp "${DISTDIR}"/* ... -- this can actually copy *all* files ever downloaded on the system 2. cp "${DISTDIR}"/${P}-* ... -- you can't really rely on DISTDIR containing no other file with this prefix.
This may even have security implications if the glob matches a file where Manifest checksum verification failed and that portage has stored in DISTDIR under a *_checksum_failure_* name.