If one is unlucky enough and his GENTOO_MIRRORS don't have the distfile for media-libs/jpeg-compat-6b-r1 it will fail to merge, as the "original" URL included in ebuild (ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6b.tar.gz) does not longer exist (ie. 404). Proposed workaround: include a working URL, even if not from the original maker's page, for example ftp://ftp.uni-leipzig.de/pub/linux/ftp:robot.informatik.uni-leipzig.de/Sources/System/Libs/jpegsrc.v6b.tar.gz (this is the first found through google). Proposed solution: ask upstream not to remove older vesrions.
Fixed the SRC_URI to point at mirror://gentoo/