When a user attempts to update their installation by running emerge -vuND world (after, of course, emerge -pvuND world), portage should check if the lower order packages (those that will be installed first) either satisfy or break the dependencies of higher order packages.
For example: say I've got openal-20050504-r1 installed alongside freealut-1.0.1 (it probably shouldn't happen since freealut depends on !~media-libs/openal-20050504, but somehow it did)
When I run emerge -pvuND world, portage tells me that openal-0.0.8 will be installed first, and that freealut will be installed a bit later. I am notified however that freealut can't be installed because media-libs/openal-20050504-r1 is blocking it.
I *should* be able to just run emerge -vuND world, and let it intelligently realize that "well, yeah freealut doesn't like openal-20050504-r1, but it does like openal-0.0.8, and since we're installing that first, this update should work just fine."
*** This bug has been marked as a duplicate of 1343 ***