Hi, after doing 'emerge clean' all packages which where outdated were also removed from the world file,e.g. i had 2 qt versions installed,so the older one was unmerged,but qt was completely removed from the world file (/var/cache/edb/world).
Um, it's possible that an older version of Portage did this, but all current versions of Portage don't modify the world file when things are unmerged or cleaned. I'm closing this bug; re-open with more info if you continue to have this type of problem.